Join Deke McClelland for an in-depth discussion in this video Device-dependant RGB and CMYK, part of Photoshop CS3 Mastering Lab Color.
In a previous exercise, I explained why you need three dimensions of information in order to express color inside of any piece of digital software or hardware whether it's Photoshop or Illustrator or any of those or digital camera, anything like that needs three dimensions of color and I showed you that using an HSL diagram, a diagram that involved the Hue, Saturation and Lightness layer mixed together. The thing is there is no HSL color model inside of Photoshop. If you go up to the Image menu and you choose Mode, which is how you get to the various color models, you'll see that there is no HSL. There used to be in the old days back in Photoshops 1 and 2 but there isn't anymore.
Instead what we have is our RGB Color, CMYK Color and Lab Color. Now, I'm going to show you how the first two work inside of this exercise, RGB and CMYK, and then I'll show you how Lab works in the next exercise. Now we happen to be working inside of a Lab image at this point, the one I have open, because Lab encloses all of the colors that you can represent using RGB and CMYK, at least in the theoretical universe here. So the name of the image that I have open, this Lab image that I was telling you about ,is The Big Three.psd, the big three meaning the big three color models RGB, CMYK and L-A-B, and it's found inside the O1 What It Is folder.
I have got a bunch of layers set up inside of this file and I have some Layer Comps as well. Inside the Layer Comps palette, you can see I have four Layer Comps. I am just showing you that because that will explain the little magical slideshow I am about to present to you. I have a keyboard shortcut that advances me from one Layer Comp to the other just so you know what's going on behind the scenes there. Alright, anyway I'm going to tab away the palette. It's not really necessary you open this file because I'm going to be demonstrating it to you here. I'm going to tab away my palettes and I'm going to switch to the Full Screen mode and you can see the title of this, it's called the Big Three Color Spaces Work and let's bring up the first color space, the one that's the most common color space you are going to work with inside of Photoshop and that's RGB.
Both RGB and CMYK are device dependent spaces, meaning that the definitions of colors vary depending on what device you are using. Now, RGB is the space used by any light capture or projection device. So anything that captures light or displays light and in the capture department that would mean scanners or digital cameras and in the display department, that would mean your monitor, your screen or your projection device, what have you. In that case we are looking at a Red channel, a Green channel and the Blue channel that interact as follows.
The darkest colors in any one of those channels is always going to be black as you can see over here on the right-hand side and the lightest color is going to be the color in question that is red, bright red is the lightest color inside the red channel. Bright green, a very, very bright green, almost yellowish green is the brightest color represented by the green channel and then a darkish blue is the lightest color that can be represented by the blue channel. So, you start with utter and complete blackness and you build color on top of that inside of RGB.
Moving right along the next guy is CMYK and CMYK is specifically designed to accommodate process color commercial printing. Many color laser printers also use CMYK. Your local Inkjet device probably uses some other combination of colors but you are basically starting with page white and you are building darker colors using inks on top of that. So you've got three color primaries right here Cyan, Magenta and Yellow which are opposites of Red, Green and Blue. So, cyan is the opposite of red. Cyan ink actually absorbs red light and bounces back green and blue, so the green and blue light mix to form cyan.
And magenta goes ahead and absorbs the green light and reflects back the red and the blue, so it looks like when the two are mixed together, red and blue full blast mixed together to form magenta, and then yellow is reflecting blue to start blue right over here and then it's reflecting back the bright green and bright red and that forms yellow. Then we have black, which is the key color, because you don't really get rich blacks out of mixing cyan, magenta and yellow together so the K is for the key color black. It's not really one of the primaries but it does hold the shadows together nicely and where any one of these plates of ink is concerned, the darkest color you are going to get out of a single plate is that color.
So, it's going to from cyan over here on the darkest side to white over here on the lightest side of each one of these channels of information. Now, the problem with rendering colors in either RGB or CMYK is that it is device dependent. So one screen for example in the case of RGB, one screen is going to show you colors differently than another screen and in the case of a printer for example, one printer is going to show you an image differently than another printer. So, if you mix a color using specific red, green and blue values, those color ingredients are going to come out differently on one screen than they come out on another.
The idea behind Lab is that its device independent so that you can define a specific combination L-A-B values and you're going to get a specific color, no matter what, you're going to get that color on any screen or any output device and we'll see how Lab works in the next exercise.