Well........If you want your hopper to look really cack in 6 months time then do as martin says
otherwise check out this link
http://www.p8ntballer-forums.com/vb/showthread.php?t=53982
Halfords car paint looks good when you've just done it but it will flake and fall off for sure as it's a rigid paint that will break off of the surface of your flexible plastic hopper
Either "DYE IT" or use "polycarbonate model paint".
Somewhere on that thread there's a bit by me with a link to my hopper, it still looks like that today
Just to prove my point, ever seen a local boy racers car where he sprayed it himself and yet all the paints flaking off the bumpers and spoilers etc etc, it's because he didn't use a special and expensive plastic preperation so the paint simply didn't bond.
Go for the remote control car shell paint and spray the inside
Have fun and get creative