The Myth: A cool gentle fade from blue to green looks cool on a helmet.
The Reality 1: Helmets get abused and get scratched, nicked, and gouged. They are regularly sent out for x-rays to check the integrity of the helmet, and then are retouched or repainted as necessary. You can't retouch a gentle fade, hence the spray-can transition of the Jags' ugly old helmets. It might happen in college, but they have different rules around helmets.
The Reality 2: Looks great side-on, but how often do we see players only from that angle? From the back at home it would look like a green eraser on top of a navy blue pen, or a World of Warcraft character stuck with Helm of the Claw.