AJ: You sure that's him?
Medic: *whisper* yes, he's turned grey.
AJ: he's not grey, he's white.[i]

[i] Turning white is a process known to some inhabitants of the Mirrorlands. More accurately it can be described as "losing colour". In the past, everybody who went to the Cube for the first time either died there (which isn't very lethal due to how the Cube works) or received serious injuries, and came back with white hair. Most people who lost their colour can't interact with colour-coded Mirrorlands' regions at all for some time until they recover.

However, Carl is known to switch to white from time to time, it usually comes with a personality change, and he can completely ignore all the rules in that state. Bella seemingly went through something traumatic and turned white in the process, but hasn't recovered so far. There's a theory that since she found a new at the time and not yet colour-coded History region and made it her home, she broke this process and had to make a new life for herself. Travis went white while still on the Cube, at the end of his first trip which lasted a couple of Cubean centuries: he finally tried to escape, went missing, and returned home looking normal but completely insane personality-wise. No memories of what happened, and no idea if he ever died on the Cube, but he remembers going white prior to memory loss.

Interestingly, the same process doesn't seem to apply to Cubeans who travel to the Mirrorlands. Maybe because thanks to the lack of real physics you can't die here. You could still get scared shitless and die IRL, so don't do anything stupid, like travel to the horrorside of the Dreaming.