In australia we have gymkhanas, agriculturals, royals and HOTYs.. so im thinking a 4-H show is just an Agricultural..
Depending on what events you go in, im not sure that painting his mane will look very outstanding!! But then again, you may be setting an all new trend!! Could go either way really!!
For his mane, plait it to 3 inches and cut the ends off with the scissors pointing upwards, so it gives it the more natural look you were going for.. Are you sewing or banding? And are you clipping/plaiting his tail?
Hope all goes well!! You could try a running plait, rosettes, rosette loops, dressage plaits.. anything really!! Just remember to keep the plaits small and neat and all should be fine!!
Oh and if you're going to use blue on blu, make sure you do a reaction test beforehand!! I'd hate him to end up with dermatitis or something!!