Hey, thought I'd share my experience with tarting up my M14 with USGI Fibreglass stock. Took two hours or so, and I'm pleased with the finished results:
I used four cans of matte Krylon, in their "camouflage" series of paints: Khaki, Black, Olive and Brown. The photo here doesn't really do justice to the colours, but you get the idea.
First, disassemble the rifle, select the parts to be painted and mask off any bits of them you do not want painted at all (sling mounts, scope controls and glass, etc.). Do whatever sand-and-prep you want.
Then spray the whole thing Khaki.
Wait 10 minutes (Krylon is touch dry by then), then mask off the shapes you want to stay Khaki. As you'll see, I wanted a kind of angular look so I used randomly-shaped straight edged patterns. I just took a roll of 2" masking tape, chopped it into lots of little pieces, trapezoid, square, rhombus, whatever, and stuck them together to make the shapes.
When you've masked off your khaki shapes, spray the whole thing black.
Wait ten minutes for the black paint to dry, then, without removing the (now covered in black) masking tape shapes, add another set of shapes for those areas you want to stay black. They can overlap the already masked-off shapes (and in fact it looks good if they do because the black will come out 'behind' the khaki).
Then spray the whole thing green, and (you guessed it) wait ten minutes and add another set of masking-tape shapes (you'll be cutting more while the paint dries) for the areas you want to stay green.
Finally, spray the whole thing Brown.
Wait ten minutes, then peel off ALL the masking tape (except the very first masking tape you put on the 'do not paint' areas) at once. Voila, you will see your magnificent pattern emerge.
To finish: Allow the paint to set for an hour, and then lightly rub it with very fine (0000 gauge) steel wool. This will take the 'raised edges' off the shapes left by the masking tape.
Allow the Krylon to cure for at least 24 hours and preferably several days or a week, and then use a satin or semi-gloss clearcoat (I used Varathane - the real stuff, not the scent-free water-soluble 'interior' stuff) to harden it (Krylon is great but soft and scratches easily).
Finally, remove the masking from the 'do not paint' areas and you're done.
Final tip: it's not hard to make the shapes from the stock 'carry over' onto the handguard. Just line them up together when you're applying the masking tape.

I used four cans of matte Krylon, in their "camouflage" series of paints: Khaki, Black, Olive and Brown. The photo here doesn't really do justice to the colours, but you get the idea.
First, disassemble the rifle, select the parts to be painted and mask off any bits of them you do not want painted at all (sling mounts, scope controls and glass, etc.). Do whatever sand-and-prep you want.
Then spray the whole thing Khaki.
Wait 10 minutes (Krylon is touch dry by then), then mask off the shapes you want to stay Khaki. As you'll see, I wanted a kind of angular look so I used randomly-shaped straight edged patterns. I just took a roll of 2" masking tape, chopped it into lots of little pieces, trapezoid, square, rhombus, whatever, and stuck them together to make the shapes.
When you've masked off your khaki shapes, spray the whole thing black.
Wait ten minutes for the black paint to dry, then, without removing the (now covered in black) masking tape shapes, add another set of shapes for those areas you want to stay black. They can overlap the already masked-off shapes (and in fact it looks good if they do because the black will come out 'behind' the khaki).
Then spray the whole thing green, and (you guessed it) wait ten minutes and add another set of masking-tape shapes (you'll be cutting more while the paint dries) for the areas you want to stay green.
Finally, spray the whole thing Brown.
Wait ten minutes, then peel off ALL the masking tape (except the very first masking tape you put on the 'do not paint' areas) at once. Voila, you will see your magnificent pattern emerge.
To finish: Allow the paint to set for an hour, and then lightly rub it with very fine (0000 gauge) steel wool. This will take the 'raised edges' off the shapes left by the masking tape.
Allow the Krylon to cure for at least 24 hours and preferably several days or a week, and then use a satin or semi-gloss clearcoat (I used Varathane - the real stuff, not the scent-free water-soluble 'interior' stuff) to harden it (Krylon is great but soft and scratches easily).
Finally, remove the masking from the 'do not paint' areas and you're done.
Final tip: it's not hard to make the shapes from the stock 'carry over' onto the handguard. Just line them up together when you're applying the masking tape.