Guys, think this through he's talking about a mouse and keyboard, they're not going to be prepainted...
You'd be better off using acrylic paint as that's not going to eat through the plastic on the mouse/ keyboard, however if you've got a small piece of the material (even if it's just in a part of the mouse/ keyboard that you don't use) that you can test the spray paint on then give it a try (it can eat holes in certain plastics hence the caution).
Matte is going to be the finish you want for it to have a military look (gloss reflects light, shine is one of the things troops look for identifying enemy positions).
As for the case: Remove all components
If the case is painted anything other than white I'd suggest removing all the paint using ethanol or something similar to prevent the darker paint showing through, the alternative is to double layer the white paint.
Spray paint it white, then cover it in paper with holes cut out in the shapes that you want the next colour in.
Spray paint it in the next lightest colour, adjust paper to have shape arrangement that you want.
Spray paint in next lightest colour, continue as before.
Leave the drying time for the paint in between each stage.
Test the paint on a concealed section of your case before you paint the whole thing, leave for a couple of days to see if there are any ill effects.
If you use the incorrect paint and it eats holes in your case that is not my fault, these instructions will work provided you use the correct paint.