The Basics – terminology
What is a color?
Color is our vision system’s perception of the physical property of light at different wavelengths. At the back of an eye in the retina are three kinds of “cone” receptors that are sensitive to the colours of Red, Green and Blue (RGB) and the “rods” that take over in low light conditions. There’s only one kind of rod, thus our color vision fades in darkness.
See how our brain mixes all the visible colors with only those three kinds of cones..
Human eyes are less sensitive to blue than to red and green
We see colour in Red, Green and Blue values but that does not mean that we are therefore equally sensitive to all three. We have evolved on a planet awash with blue light and as a result evolution has given us eyes that are only half as sensitive to blue as they are to red and green to help compensate. This 40% Red, 40% Green, but only 20% Blue sensitivity in the cones at the back of the eye results in us seeing blue as a dark colour. This is of benefit in nature, but with the invention of printing it has resulted in a real problem. We just can not produce blue dyes, inks and paints that are of a strong enough colour to compete with the extra sensitivity we have to red and green. For some reason we also have a stronger reaction to green than to red. The combination of RGB makes white, but of the three, green is the brightest, followed by red, and blue being the darkest.
R G B will mix into C M Y
The secondary colours of Cyan, Magenta and Yellow (CMY) are made by mixing two primary colours each. Mix red and blue and you get magenta, cyan is a combination of green and blue and if you mix red and green you’ll see yellow. Because for each the cones of two primary colors are used magenta, cyan and yellow look lighter than red, green or blue. Yellow looks the lightest because it uses the cones of green and red combined where we have the most receptors.
Where the heck is Magenta in the color spectrum?
Magenta is the only color that does not exist as a single wavelength of light. Our brain makes it up to have something to see when blue and red are mixed.
Read more about it here:
A beam of white light is made up of all the colours in the spectrum. The range extends from red through to violet, with orange, yellow, green and blue in between. But there is one colour that is notable by its absence. Pink (or magenta, to use its official name) simply isn’t there. But if pink isn’t in the light spectrum, how come we can see it?
Already Sir Isaac Newton noticed its absence.
It is an “extraspectral” color. Sir Isaac Newton noticed that magenta did not exist in the spectrum of colors from white light when he played with prisms. But when he superimposed the red end of the spectrum on to the blue end, he saw the color magenta (this can be done with two prisms to make two spectral spreads, “rainbows”)
How many colors can our eyes and brain perceive?
Scientists from the International Commission on Illumination (CIE) conducted real-world experiments in the late 1920s to answer this question—about 10 million, they concluded. They took this data and plotted it onto a graph in 1931, now known as the CIE 1931 Chromaticity Diagram.
This diagram and the CIE XYZ color space and later on the CIE LAB color space from 1976 are the foundation from which digital color is built. Think of it as a map, where every humanly-perceivable color can be found.
A color model is an abstract mathematical method for describing a color based on its component properties.
There are several color models like RGB, CMY(K), (CIE)LAB, HSL and HSV
The RGB color model is based on three primary colors of light that mix additive.
The CMYK color model is based on subtractive mixing of pigments and is used for printing with 4 colors on paper.
Watch the video to get more info on what the difference between RGB and CMYK is:
Autotypical Color mixing in Offset printing
in an offset CMYK print, color images are produced using a four-color process with cyan, magenta, yellow and black inks: halftone dots are sized depending on the desired color tone. When overprinted, some of the dots corresponding to the individual colors are adjacent to one another, while others partially or entirely overlap. In this case the resulting color is the result of both subtractive mixing and additive mixing, since when looking at an offset-printed item at a normal viewing distance, our eyes are unable to distinguish the individual dots. This combination of additive and subtractive mixing is called autotypical mixing.
CIELAB and CIE XYZ are similar color models designed to approximate human vision. Because these color models include so many colors, they are both used when translating from one color model, such as RGB, to another, such as CMYK. These are referred to as profile connection spaces (PCS). For instance, Photoshop uses CIELAB as a reference color space when it converts from one RGB profile to another RGB color space. Photoshop uses CIE XYZ when it converts from the RGB color mode to the CMYK color mode. It’s possible to use the CIELAB color space for image editing in Photoshop, although few choose it for that purpose since it is not as easy to understand as the other color models.
A color space is the set of colors that can be displayed or reproduced in a medium (whether stored, printed or displayed). Within each color model there are many different color spaces. Some RGB examples are sRGB, eciRGB, Adobe RGB, Pro Photo RGB, NTSC, PAL. Or in CMYK there’s ISOcoated, ISOuncoated, SWAP, etc.
Watch this video to see how RGB color spaces are represented in the CIE chromaticity diagram and why with only three colors you can never show the entire range of visible colors.
For every Color model there are different color spaces, each with their own advantages and disadvantages, but we can sometimes compare them by how big of an area they cover on the map (their gamut). Bigger gamuts cover a larger range of possible colors.
The terms color space and color profile can often be used interchangeably, since a profile is a description of a specific color space.
What’s a color profile?
A color profile is the numerical model of a color space. Operating systems and programs need to have access to a profile that describes the meaning of the color values in order to interpret and display the color correctly. Also if you save a file most of the times you will save it with the intended output color space as an embedded profile.
Commonly used color spaces/profiles
Working with computer imaging we will have to deal with a handful of different color spaces and profiles in our work. In general, these can be divided into three: Working, Output and Device spaces. Let’s take a look at each group and list some of the options you’ll see in each.
Working color spaces are well suited to image editing tasks such as color and tone adjustments. Ideally, these are large color spaces, offering the photographer the ability to choose between a wide gamut of colors. There’s no single ideal color space but there are a few very good choices.
- ECI RGB v2 – This color space is the recommended space in the Metamorfoze Preservation Imaging Guidelines and the only one allowed at the highest level of those imaging standards. This profile is the most likely to produce the same result across different color engines. Although white is not neutral (it has a slight tint at 255, 255, 255), it is the only profile that avoids all the other compatibility issues. ECI RGB v2 has a usefully wide gamut that extends beyond AdobeRGB in certain hues, however, it cannot quite represent saturated blue and magenta hues that sRGB can. Nevertheless, it is particularly suited to modern printers and, with correct exposure, uses bit-depth efficiently, making it one of the most suitable color spaces for 8-bit/channel images
- Adobe RGB (1998) – As the name implies, this color space was created by Adobe in the late 90s, when Photoshop implemented full color management. Although Adobe never intended this color space to be the universal standard, it is widely supported. Most DSLR cameras offer this as a color space choice for JPEG creation. While Adobe RGB does not contain as many colors as ProPhotoRGB, it’s easier to use and a very good choice for both 8-bit and 16-bit image editing.
- ProPhoto RGB – This color space was designed as a universal standard for high-bit image editing, and includes all the colors that the human eye can see. ProPhoto RGB is a very popular color space for experienced Photoshop users. Because the space is so wide, it’s not appropriate for 8-bit images. ProPhoto RGB is not always the best choice for a working space. Because 15% of the color space is beyond the range of human vision, color mapping is happening in ways that are impossible to see. Some images will suffer when they are converted from ProPhoto RGB to a CMYK space because these out-of-gamut colors map to the destination space in unwanted ways.
- CIELAB – This color space is used internally by Photoshop during color space conversions. Some people have found interesting ways to use CIELAB space to manipulate images, since the luminance is totally separate from the color information. However, CIELAB is not an easy space to use since it’s not intuitive for most people. Images that are in CIELAB space also don’t have full support in Photoshop for adjustment layers and other non-destructive imaging tools.
- sRGB – This color space is a small color space – it’s often thought of as a lowest common denominator. sRGB is very similar to older monitor spaces and, in fact, it’s common for unmanaged computers to assume that an image is in the monitor color space. This makes sRGB a good choice to send to unknown users. At the moment, sRGB is the only appropriate choice for images uploaded to the web since most web browsers don’t support any color management. Additionally, sRGB is a very good choice for images sent to minilabs, especially if there is no custom profile available. Because sRGB is not a wide color space, it’s not appropriate as a working space.
- CMYK profile – When you send an image off for CMYK process printing you will need to convert your image to CMYK. If you’ll print on coated paper you should use either ISO coated V2 or the newer version PSOcoated_v3. For uncoated paper there is ISO uncoated and PSOuncoated_v3. Check with your printer what color profile they want for the printing process to run as color-accurate as possible.
Photographers will most commonly run into device-dependent color spaces when profiling monitors, desktop printers or sending images out for CMYK printing.
- Monitor RGB – Modern monitors include a factory-created profile that is loaded into the monitor’s firmware and is communicated to your computer via the monitor connection cable. If you want your monitor to do the best possible job reproducing your images, you should create a custom profile for it.
- Desktop printer profile – Your printer comes with profiles in the driver software. You can create a custom profile for your printer if you want to maximize its color fidelity.
Converting Colors VS. Assigning Profiles
Sometimes we will need to switch profiles like f.E. from Adobe RGB to sRGB. In Photoshop there are two options for this and they are significantly different. Convert to Profile and Assign Profile.
Assigning a profile
As we now know a color profile is the numeric representaion of colors in a color space. That means, that for every pixel in an image you computer will read its RGB value and then decide, by looking into the profile, how to display this value. Because every color spaces size (gamut) is different the same RGB numbers will point to differently looking colors within its range. Try it out and create a file and fill it with a medium solid color. Then go to Edit → Assign Profile and see how the look of the color will change dramatically with every profile chosen, while the RGB value stays the same.
To sum it up: Assigning a profile changes the interpretation of RGB values and thus the look of the image. RGB Numbers will not change. Profiles can be non-destrucively assigned without actually changing the image data.
Convert to profile
This is like the opposit of assigning. When you convert to another profile, Photoshop will try to keep the look of the color as closely as it can, while changing its RGB value to one that is inside the new profiles gamut. Because some profiles are smaller than the other, PS will need a way of knowing what to do with the RGB values that may be outside the new profiles color space. This is what you can choose under Intent in the Conversion options.
For most situations Relative Colormetric with Black Point Compensation turned on is the best solution.
Long Story short
When we work on a computer we will use RGB color spaces that provide a big enough gamut for us to get our images full potential (ECI_RGB or Adobe RGB).
If we want to save images to the web we need to convert them before we do that to sRGB so that they will display as intended in any browser. Embedding sRGB or not does not make a difference because most browsers don’t have color management.
For printing purposes we convert our images to CMYK. The output profile in this case depends on the paper that will be used for printing and also the print-shops preferences.
Best practice: Work in a big RGB space, always keep your original work in your working RGB and only create flattened copies converted to the chosen output profiles.