Substrate colour

Well, there's been lots of discussion about substrates here. How about the
colour of the substrate? Does anyone have any ideas on whether a dark or
light substrate is better? I have a friend who's aquarium has white gravel
and one who's aquarium has black gravel. Both tanks look terrific. What are
the pros and cons of dark or light? I ask because I've been thinking of
laying some darker gravel over my light brown variety.

in lovely Vancouver -- about a perfect 75 F.