Why is brown fat brown?
Brown fat is so-called because it contains a large number of mitochondria, certainly greater than its white fat counterpart. The reason for the excess in mitochondria is to participate in thermogenesis (incidentally, brown fat is also called thermogenic adipose tissue). Nearly all mammals have brown fat (excluding pigs), especially those that hibernate. Brown fat require larger numbers of mitochondria in order for the fat to be used in beta oxidation (which is carried out directly by mitochondria), and subsequently produces heat via the electron transport chain. This also requires greater access to oxygen from the blood, and as a result, greater capillary density also darkens the appearance of the gross tissue. Mitochondria themselves are dark because of the iron content; vitamins B2, B6 and B12; as well as the pigmented flavins (FADx).