Why should one only eat M & Ms that are primary colors?

I went to a golf tournament party today. Great fun.
My host, one of my good friends who is a wealthy attorney, and who I admit is a bit strange (but a great guy otherwise) only eats M &Ms that are the primary colors.
I played the straight man and asked him why.
He said that he really did not know, but he was counting on me to make a rational argument why he should, so that in the future he could answer the question.
I suppose I can come up with something, but before I do…
What would you say?