The secret to the custom mags is RIT dye. Available at most grocery stores in the laundry detergent isle. Pick your color then follow the following instructions:
Take a 2 quart pot with about 1 to 1 1/2 qtrs of water and bring it just to a boil.
Add one table spoons of RIT dye and one table spoon of salt.
Remove pot from heat and add your disassembled plastic parts.
Takes about 4-5 minute for color to soak in. Check progress as you go. Long time equals darker color. Rinse parts with water and dry. You can re-dye the part if they are not as dark as you want.
Re - assemble mag and your good to go !
FYI, this dye will add color to a white sink. Be sure your wife is not home and that you have bleach on hand to clean up the mess.
Dye sets into the plastic. Once it is pulled out of the dye bath and rinsed off it's in there for good. I have had no problem with it leaching out or problems with oil reacting to the dye. I just rinsed the parts well to make sure all the salt was removed from the plastic parts and re-assembled them. They look cool and makes it easy to color them different for different loads and you can still see how many rounds remaining in the loose mag verses the black ones where you can not see anything but the next round.
Kevin - I would not thing you could do anything to the black ones since the plastic is already black and the dye would not make a difference.
I have seen others color the clear mags blue, yellow, green, smoke and orange. Looks like them old candy suckers on a stick you had when you were a kid !