I'll show two methods of making C&C screenshots here:
This method is fairly simple. I made a pack of colour palettes, so all you need to do is paste the screenshot in PSP and load the correct palette into it, in the correct way.
If you don't have PSP, I got an old trial version of PSP3 here. This is a 16-bit program though, so it doesn't run on 64-bit Windows systems, and it can't save filenames which are longer than 8 characters or which have spaces in them. You can ignore the 30-day trial period message though; the program doesn't stop working after that period.
Download the Paint Shop Pro palettes pack here. Unzip them somewhere so you can access them later.
As for the actual screenshots... assuming you went ingame, and pressed Print Screen at some point, here's what you have to do. Open the main menu ingame, press [alt]+[tab] or [ctrl]+[esc] or [Windows key] to switch out of the game, launch Paint Shop Pro, and paste the screenshot. The result will be mostly black and dark purple.
Now, to fix the colours:
Overview of the palettes:
Note: you can't use this method to take screenshots of cutscenes, unless you go find the vqa file in XCC Mixer, copy it as pcx or png frames, then open a frame of the same scene you print-screened in PSP, save the palette to a new file and then load that palette into your pasted screenshot. Kinda ridiculous process imo, since all it does is add the interlacing :p
This is pretty much the "quick & dirty" method. It doesn't take any effort at all, but it kinda screws up the colours. The advantage is that it also works for videos though.
Open MS Word, and open the Office Clipboard bar. In older ones you can do that just by selecting something and pressing CTRL+C twice, but in Office 2007 you have to open it manually, apparently.
Leave Word opened when playing the game. You will now see that screenshots made with Print Screen will show up correctly.
However, in reality this overwrites the first and last 8 colours of the palette by the Windows standard 16-colour palette. The Office Clipboard compensates for this decrease in colours, but it's still noticeable that the green health bars of units are not the same green they should be.
I don't advise using this method, but, well, it works, so it's worth mentioning. But I wouldn't use it to make showcase screenshots for anything.
Notes on using MS Paint for saving: