Dealing with Untagged RGB Files
"Untagged RGB" isn't just another color space. Indeed, it's the lack of one. A file that isn't tagged as being in any specific color space still has red, green and blue values for each pixel of course, it's just that a program reading it doesn't have any way to know what those numbers mean.
I've written before about the differences between sRGB and Adobe RGB. Each of them uses the same system to describe color since they share the same RGB color model. But they differ in terms of gamut, and a given set of red, green and blue numbers in one doesn't mean the same color as it does in the other. So what would happen if you had a file you knew was either sRGB or Adobe RGB, but you didn't know which? How could you tell which it was?
Before you can solve this, the first thing you need to know is that the problem even exists. Many programs ignore color profiles completely. But even among those that honor color management, many will silently just display untagged files either as sRGB or as if they were stored in the same color space as your monitor profile.
The Color Settings in Adobe Photoshop let you tell the program how you want to handle untagged files. Selecting the checkbox at the bottom of the Color Management Policies section instructs Photoshop to warn you when you open a file without an embedded profile. When this happens, you will see a dialog with three choices asking you how you want to handle it. "Leave as is (don't color manage)" will effectively disable color management for the image, causing it to be displayed simply using your monitor profile. "Assign working RGB" instructs Photoshop to treat the colors in the file as being in the color space of your working space profile. "Assign profile" is effectively the same thing except that you can pick any profile not just your working space. For convenience, this third option also gives you the option of converting the newly assigned data to your working space once the file has been opened.
If you ask whoever you got that image from what color space it is supposed to be you may be able to pick the correct color space when opening it. But they may not know either. I mean, if they knew, they probably would have embedded the profile when they sent you the image. There are exceptions of course, but quite often "untagged RGB" really means "unknown RGB."
If you don't already know which color space to choose, the problem is that both of the Assign options give you nothing to go on to guide you. If only you could at least see the image, you could pick a profile that rendered the image in a pleasing way. Since you can't see the image yet though, go ahead and choose the "leave as is" option when you open an untagged image. We can deal with the profile problem more easily once the image is open.
After opening the untagged file, select Edit >> Assign Profile. Go through the list of available profiles, trying each one. With the Preview option checked, Photoshop will show you the effect of each chosen profile on your image. You can try different profiles until you find one that makes things look right. If the originator of the image used something odd, you may still have a problem, but in most cases you should be able to find some profile on your system that will give you a good looking image.
The effect of assigning different profiles. Which looks right to you?
Once you have assigned the profile that seems best, go to Edit >> Convert to Profile to convert things to your working space profile. This will simplify things later and keep things consistent with how all your other files are saved.
If you don't check the box for "Missing Profile: Ask When Opening," Photoshop will silently treat the data as being in your working space. That is, unless you set the RGB Color Management Policy to "Off" that is, in which case every image will remain unmanaged. Either way, this is not what we want.
What Photoshop ought to do is show you a preview image as part of the "missing profile" Open dialog to guide you in the same way Edit >> Assign Profile does. Maybe some day Adobe will see fit to do so, but until they do, at least you can tell what you are doing once the image is open.
Are you listening Adobe?