Use the color's 3-part RGB (Red-Green-Blue) code for the following equation:
[Blue] + 256 x [Green] + 65536 x [Red] = [Z] (the number you will need for the command)
If you want Red, the 3-part RGB code will be 255 000 000
You would enter it as:
000 + 256 x 000+ 65536 x 255 = 16711680
[Z] = 16711680 (for Red)
Therefore, the command for getting a bright red leather cap would be:
/give <Player Name> leather_helmet 1 -0 {display:{color:16711680}}
Use this page for more RGB codes.
(Also, you cant change the outer layer, I'm assuming you mean like the brown stripe on the cap, the knee-pads on the pants, or the bottom of the boots.)
-SmashPortal, King of the Cave Spiders