Posted this on the same thread, but on the G37 forums. Mine got like that after repeated sessions on the track - it's just heat discoloration. I assume the silver paint isn't as heat resistant as the red or grey paint on the Nissans, or just a by-product of them being silver.
Per the recommended advice above, I'd probably change the brake fluid. I run Motul RBF600 in my car simply because it sees the track, but you probably won't need that.