Give it a few more days. If he was grey then got brighter he'll probably shed soon. Make sure your husbandry is right and nothing is around to stress him out.

I'd offer him food on the next scheduled feeding day. If he eats, I'd think it a fluke thing and he should be fine. If he doesn't, I'd personally think about a vet visit.

This is what I would in general for any snake. I do not have BRB so do not know specifics of what could be wrong.