when 우유 is processed into butter, the 버터 cream separates from the watery liquid (which is actually sold as skim milk). the 버터 cream contains all the fat which has a high concentration of vitamin A. Vitamin A has a natural yellowish tint. When churned, it is oxidized and aerated, and turns even yellower and another separation occurs when the fat lumps and the 버터 우유 (sold as, 당신 guessed it "buttermilk) is squeezed out. how do I know this? I grew up in a dairy farm in Wisconsin:)
