What is the proposed selection pressure that leads to the distinctive coloration of Orca whales? I can find nothing in the literature.
You can find a discussion of this topic in the literature:
Mitchell, Edward. 1970. Pigmentation pattern evolution in delphinid cetaceans: an essay in adaptive coloration. Canadian journal of Zoology 48(4): 717-740.
Perrin, W.F., 1972. Color patterns of spinner porpoises (Stenella cf. S. longirostris) of the eastern Pacific and Hawaii, with comments on delphinid pigmentation. Fish. Bull, 70(3), 983-1003
Caro, T., Beeman, K., Stankowich, T. and Whitehead, H., 2011. The functional significance of colouration in cetaceans. Evolutionary Ecology, 25(6), pp.1231-1245. http://web.csulb.edu/~tstankow/Caroetal2011.pdf
And related helpful article:
Evans, W.E., Yablokov, A.V. and Bowles, A.E., 1982. Geographic variation in the color pattern of killer whales (Orcinus orca). Report of the International Whaling Commission, 32, pp.687-694. http://www.kasatka.com/kirsten/ColorVariation.pdf
I'll let you explore these and their citations.
Remember, Google Scholar is your friend.