i choose thread colors to go with the top & bottom- if white works on top & blue on bottom-then that's what i use.
or what ever. often i will (try) to make them the same (value) but often they are different colors- once in a great while i will use the same top & bottom - if the backing goes with the top well enough for the same to work.
often when people say you should use the (same thread) they mean* type/fiber content/weight*---not necessarily the same color.