Photoshop Working (Space) Behind the Scenes
People trying to make sense of the color management settings in Photoshop often puzzle over the choice of RGB Working Space. When they change it from one flavor of RGB to a different one, images don't seem to look any different, which merely serves to reinforce the idea that color management is a waste of time. But while they're right that images look the same, there's more to color management than meets the eye.
Color spaces and color profiles are everywhere, but each of them serves a different purpose in making this whole color management thing work. When you print something, the printer profile for your printer model, paper and ink choice gets used to compensate for the inherent imperfections in how your printer hardware reproduces color. Your monitor profile handles a similar job when you view images on your monitor. No monitor can accurately reproduce color without at least some help, and a monitor profile tells your computer's operating system exactly what help it needs. If you use a slide scanner it too can benefit from a bit of help in the form of instructions embedded in a color profile based on how it sees color.
But among all of these, the role of the Photoshop Working Space profile is different and can therefore seem puzzling. What exactly is it translating or correcting color from or to? Well, ... that would be your image file itself. Or not. It depends.
If you hover your mouse over the words "Working Spaces" in the Color Settings dialog, the thoughtful folks at Adobe will inform you in the Description area that "The working space specifies the working color profile for each color model. (A color profile defines how a color's numeric value maps to its visual appearance.) The working space is used for documents that are not color-managed, and for newly created documents that are color-managed." To understand what a working space is for, let's take this one sentence at a time.
A "color model" is a way to describe color. It doesn't speak to exactly what color is being described, but just to how they get described. For instance, the RGB color model uses three numbers, one each for red, green and blue. Every variation on describing color by means of increasing values of red, green and blue constitutes an instance of the RGB color model. Then there's the CMYK color model that uses four numbers to specify cyan, magenta, yellow and black (K). If a color system uses one number each for these same four colors then it is an example of the CMYK color model. And so on.
But while the RGB color model says that color is described in terms of its red, green and blue components, it you have to choose a specific RGB color space to know exactly what color any set of three numbers actually represents. Some RGB color spaces cover move colors than others or are skewed in various ways to fit various needs so that while (200, 50, 100) describes a color somewhere in the magenta/salmon range, exactly what specific color won't be the same in all of them. Both the sRGB and Adobe RGB color spaces are members of the RGB color model, but Adobe RGB covers a much wider "gamut ," or range or colors, than does sRGB.
So if this is true, why doesn't changing your Photoshop RGB working space from sRGB to Adobe RGB change the way your images look? This is where the last sentence of Adobe's description comes into play. Documents themselves contain numbers representing color, so if like most photographers you work in the RGB color model, your image files are full of numbers representing combinations of red, green and blue. And if like most photographers you care how your images look, it's not enough to know that these numbers mean some color, you want those red, green and blue numbers to represent specific colors. If your images changed appearance radically when you closed them and reopened them you'd be a bit upset. Thus, your RGB images are not just RGB color model images, they have a specific RGB color space embedded so each set or red, green and blue mean something specific. When you reopen an image saved in sRGB it doesn't matter whether your working space is set to Adobe RGB or sRGB. The colors in that image get interpreted as sRGB. If they didn't, you'd be upset.
The value you pick for your working space profile comes into play only when you either open an image that doesn't have an embedded profile or you create a brand new image. Most photographic images these days do have embedded profiles. Your camera probably puts one there for you so that your images have a fighting chance of looking correct when you open them. Web images often don't have a specific profile as they are assumed to be sRGB, but most other images do these days. Open one up in Photoshop and the embedded profile will get used even if you didn't know one was embedded. And most images photographers actually care about come from cameras rather than being created from scratch in Photoshop, so the occasions when your choice of RGB working space actually gets used might be few and far between.
Once you open an image though it may get converted from the embedded color space profile to your working space for you based on how you have your Color Management Policies set in Photoshop. But when it does, the RGB numbers in that image get updated so they still represent the same color described by that embedded profile. Depending on your choice of working space profile, not all colors can be converted exactly of course since not all color spaces cover the same gamut, where their gamuts overlap, you will end up with the same color you started with even as the exact numbers used for each color change.
It's generally a good idea to use a wider gamut color space such as Adobe RGB in part to make sure all the colors in your image files will fit in your working gamut if you choose to convert them. You don't need to convert them though. If you set your RGB Color Management Policy to "Preserve Embedded Profiles" you can work with images just fine regardless of what color space they may be in. Even if you open two files in different color spaces at the same time, each uses what is embedded in it without any problems. Adobe treats each file separately even though they get displayed on the same monitor. In this case, your choice of RGB working space becomes fairly irrelevant since it never gets used. My own preference is to set the policy to convert to my working space when opening, but there are no hard and fast rules that work for everyone.
So the bottom line is that your RGB working space may or may not mean much, and even when it does, Photoshop obligingly converts RGB numbers for you to keep your images looking as close as possible to how they are supposed to. Since, as I say, otherwise you'd get upset. And Adobe doesn't want to upset you.
By the way, one profile you don't want to set your working space to is your monitor profile. If you profile your monitor (and you are profiling your monitor, aren't you?) let the software you use for that deal with the resulting profile. Your image files shouldnt be constrained by that profile. Otherwise, you'll be in a real predicament when you buy a new monitor.