I try to keep the back and top threads in the same color family especially when working with cotton battings. There isn't enough space for the threads to interlock and you can get a "pop" of the bottom thread to the top or vice versa. If thread colors are closely related this is less noticeable. It's why I like quilt tops that co-ordinate with the backing. I always like to talk this over with clients. If its possible pooling some thread across the quilt can help you choose a color.