I've maintained several planted discus tanks over the past few years, and have not had any problem with either fish colors darkening, or peppering, using a dark background, AS LONG AS the substrate is very light-colored.
My tanks have usually had a dark blue backgound, using white pool filter sand as the substrate.
Have a look if you care to:
The first set of pics has a group of 3" juvie Red Snake Skins, while the second set is an assortment of older juvies and near adults.
http://s1105.photobucket.com/albums/h357/discuspaul/3RedSnakeSkins
http://s1105.photobucket.com/albums/h357/discuspaul/Sept2011