The Vast Uncharted Reaches of Color Space
Your monitor can only display a certain range of colors. Likewise, your printer is limited by the formulation of the inks it prints with. So why do most of us work in color spaces like Adobe RGB and ProPhoto RGB that include colors we can't even use?
The range of colors usable on any given device is known as that device's gamut. The range of colors describable in given color space is that color space's gamut. In both cases, it's the same thing. Whether you are aware of it or not, you deal with color spaces and the color profiles that describe them every time you work with digital images. We need a system to describe color digitally, and any system has limits.
In the world of computers digital color means having to specify everything digitally. One eight-bit byte can hold a number from zero up to 255 (256 total values), no less, and no more. Use two bytes together and you can store a number from zero up to 65,535 (65536 total values). When describing color using the RGB model, we get one number of each of red, green and blue. So there are exactly 256 or 65536 discreet shades of red in any given RGB color space, just as there are that same number of blues and of greens. Multiply the possibilities all three color channels together and you come up with around 16 million or around 281 trillion total colors spread across the entire rainbow and from fully saturated hues to shades of gray, and from dark to bright. That may sound like a lot and it is, but it is a limit.
You can use those digits to accurately describe a small range of color or less accurately describe a wider range of color. You can't have both. If we limit our discussion here to 256 values (zero through 255) you can define exactly 256 discreet points from no color to lots of color along a continuum of red, green or blue. Those points could be spaced closely together and encompass a narrow range or they could cover a wider range by being spaced further apart. You can decide, and by doing so you define the gamut of what you can describe based on that choice.
Given this, it might seem prudent to make use of a color space that most closely matches what your monitor or printer can do. That is, if you can't use a specific shade, why waste space on being able to describe it?
There is of course one obvious problem with this approach. Sooner or later you will get a new monitor or a new printer. And since newer generally means better, especially in the world of technology, you will one day want colors that you might feel you don't need today. And your friends and colleagues today may already own the monitor you will lust for tomorrow. Your limits may not be their limits. If you send them any images it would be nice if they proved worth of viewing on that expensive monitor.
But beyond this, there's another inherent problem with attempting to fine tune your working profile to that of your monitor or printer. Device profiles are inherently quirky and irregularly shaped. Printer inks and monitor liquid crystal displays conform to the rules of chemistry and other aspects of the real world, not to idealized conceptions of describing color.
Color profiles come in two basic types: matrix based profiles, and table based profiles. Matrix profiles are based on a mathematical model defining formula for the response curves of each RGB channel as well as a white point definition and other needed values. They are small files from which your computer can generate a range of mathematically precise colors. Tables based profiles make use of a "Look Up Table" (LUT) defining a set number of specific colors. To generate colors in between these predefined colors your computer interpolates based on the nearest defined points. LUT based profiles vary in accuracy based on how many entries their tables have.
Matrix based profiles are great for defining arbitrary color spaces in a simple, accurate way. They may not correspond well to any real world device, but work great as standardized color spaces for image sharing. sRGB, Adobe RGB and ProPhoto RGB are all matrix profiles. On the other hand, LUT based profiles work better for describing the irregularities of real world devices such as monitors and printers. Virtually all printer profiles are table based.
Neither type of profile is necessarily better than the other. It's just that they generally serve different purposes. By working in a standardized matrix profile such as Adobe RGB or ProPhoto RGB you can share documents with other and be assured that your files can be utilized by a broad cross section of current and potential future purposes. By converting them to your particular monitor profile or printer profile you can render your image as accurately as practical on your particular devices.
Because these two types are different though, the shape of their gamut is unlikely to match each other. Generally speaking, you have a choice of working in a color space that is smaller than that of your monitor and printer, or bigger than they are. sRGB, Adobe RGB and ProPhoto RGB are the most commonly used working space profiles. But if your printer can print colors beyond the gamut of sRGB then limiting yourself to sRGB would mean wasting some of the potential for your printer. As such, most serious photographers have adopted the use of a wider gamut space for their work. While Photoshop gives the user complete choice of working space, you should limit yourself to either Adobe RGB or ProPhoto RGB. If you use Adobe Lightroom, you are working in ProPhoto RGB.
Working with a profile that exceeds what you can see on your monitor or print on your printer may seem odd, but it's a lot better than working in a profile that's too small.