The message
from Chris Bacon contains these words:

Nick Maclaren wrote:
White. More importantly, it should be porous to water vapour, as
sweating is the primary mechanism for keeping the brain cool.

But it's not simply perspiration from the scalp that cools
the head, is it. Leaving one ankle, wrist or whatnot(!)
uncovered would do just the same job.

You emit heat faster from the head than anywhere else except the
trunk. That's why premature babies in incubators wear hats, and
rescuers cover the head of someone who has hypothermia.

Janet.