Does white underwear keep you warm more or less than similar pieces of black color?
IMHO, white color would more "reflect" back the existing temperature of the human body. However, black would "attract" warmth of radiation coming from outside, according to what is considered as black body.
Does it also matter, whether other clothes are worn on top of the underwear, and its color?
Note: I am no expert in physics at all, so please feel free to edit this question to be more technically correct.