Personally I cant afford Photoshop, nor can I understand GIMP. The process is mostly the same for each though.
step 1 --- locate the files you want: what you will need is the *_CO.paa and the *_NOHQ.paa image of the same template
step 2 --- convert them to a .tga file, so they can be edited in your choice of photo rendering program [Photoshop, GIMP, or Paint.net(The one I use)]. to do this, go to texview, open the files there, and save them as a .tga. you probably want to keep the title the same so you dont get mixed up.
step 3 --- open the *_NOHQ files, and then greyscale, desaturate, or whatever makes them grey and uncolorful them. i personally am debating weather the inverting looks better, you can decide for yourself.
now comes the fun part: Painting
step 1 --- save your image as something different, you may want to close it and work on it later.
step 2 --- [photoshop and paint.net] create a new layer, and set layer properties to multiply. this will darken the colors you want to use, so be aware of that. use the *_CO.paa image to reference where the pain may apply, such as a vest, helmate ect.
step 3 --- once you are happy with what you want, you are now able to test this in game, nothing special is needed, you can even put your texture into a mission folder, and export to single player. a different code will be required for either process though.
-Mission --- put this into the init line of your char: this setObjectTexture [0,*path to your *_co.paa*];
-config.cpp --- in the units definition, you may place this code:
hiddenSelections[]={"camo1"};
hiddenSelectionsTextures[] = {"*path to *_co.paa"};
I have no idea if this works with ACR troops, they may not have the option.
I'd have to look into which units have that option: most vehicles, such as the Abrams have that.
hope this helps any bit
-CTF_Arora