Many famous LAers, simply refuse to use two different colors. Our own Green Fairy who does AMAZING LA work wrote on her blog once that she learned early on and tells her clients, it doesn't matter what color their back is she will always use the same color thread in her bobbin as in the top. I belive Karen McTavish and Angela Walters also go by the same rule. I am not positive on them though. I actually love the results Green Fairy gets when she uses a contrasting thread on the back.