Board Thread:Speculation House/@comment-3257939-20131111020445/@comment-10380701-20131117030931

Based on GotGame's statement, I think I semi-understand why they have different surnames.

Summer Rose and Papa Xiao Long are divorced, with Ruby going with Summer and taking up her surname, and Yang going with Papa Xiao Long.

Of course, Yang and Ruby occasionally meet up, I guess? To help cover up a few holes, maybe the times when Ruby visits them is also the time where Papa Xiao Long gives Ruby some of the advices she mentions during Volume I.

It doesn't explain their different color. But I'll chalk it up to anime logic.