Buddy had a 96' KX250 that someone had painted the frame white - used it looked like crap. Every ding, rust spot and peice of dirt just jump out at you. Looks the worst around the lower frame tubes and countershaft sprocket.
Black has some of the same issues with dust, etc. and IMHO makes you look like a member of the Metal Mulisha (not good).
He then painted it flo green and sold it, and it looked worse than white because it didn't match the plastic, and over time I've seen the flow green get lighter with UV rays, uuck.
Go with the stock green and paint it with PJ1 frame paint, OR try a metallic dark grey or silver. With PJ1 you can touch it up yourself by simply masking off any areas (motor, footpegs with aluminum foil).
My brother bought Larry Roeseler's KLX365, and Stroker had painted the frame that a dark silver metallic. Looks cool and holds up well to dirt and abuse. If anyone gives you grief, just tell them that Roeseler paints his bikes this way. BTW, grey looks best without the purple graphics, go with the new black and silver stickers.