What gives fresh milk a yellow tinge?
The main substances that cause the yellowish color of the milk are carotenoids [1]. The main carotene involved is the beta-carotene coming from the feed that cows eat.



Some studies have been carried on and it has been noticed that the milk with a more yellow tinge was collected during late spring and early summer when carotene levels are at a maximum [2] unfortunately for this reason in many products as mozzarella they add titanium oxide.
The amount of Riboflavin is too little compared to that of beta-carotene and generally gives a slightly green-ish tinge to the milk as reported here and here [3].


P. F. Fox, Fundamentals of Cheese Science,p. 13
Kosikowski, Frank V., and David P. Brown. "Application of titanium dioxide to whiten mozzarella cheese." Journal of Dairy Science 52.7 (1969): 968-970.
Marion Eugene Ensminger,Audrey H. Ensminger,Foods &amp; Nutrition Encyclopedia, 2nd Edition, Volume 1,p. 798