I saw the class banners in Evans this morning thanks to Alumnae Weekend, and it made me think of class colors mod 4. (Basically, a number c is congruent to b mod 4 if 4 divides c-b. Basically, if there's some integer k so that 4k=c-b. So 16 is congruent to 0 mod 4, 47 is congruent to 3 mod 4, et cetera.) If we assume that the colors have always alternated yellow, red, green, blue, then the blue class years are congruent to 0 mod 4, yellow to 1 mod 4, red to 2 mod 4, and green to 3 mod 4. If we think of this mod 2, the 0 mod 2 classes are sisters and the 1 mod 2 classes are sisters.
Wow, I'm such a nerd.