C'mon guys, be a bit easier on the kid, he probably can't handle the flaming.
You're adults making fun of 16-year-olds!
Anyways, you're obviously underage, OP, but a question is a question!
Step 1: Finding paint!
Get some good spraypaint. Krylon is always good. Always test the paint before you use it, and make sure it's not the sticky, shiny crap.
You don't want your gun ending up like this:
YouTube- M4a1 progress 2
Step 2: Applying tape!
Cover the parts you wish with tape. Don't forget to tape the orange tip!
Step 3: Painting!
Spray the paint in rows over the gun like so:
Do this 4 times, with about 5 minutes in between for the paint to dry thoroughly.
Let dry for 10 minutes while you go watch TV.
Come back knowing you've got a sweet painted gun for you to use