"When displaying my oil paintings for sale, what is the best color for the walls on which the paintings will be hung? Is gray better than white? Most of the paintings are medium to dark in overall appearance." -- B
The best colour is very subjective. Go with what your instinct tells you, and trust yourself. White became the norm in the 20th century, but before that quite strong colours were used. You'll see this in some big art galleries today, particularly with their more historical paintings. If you can't get to a gallery, browse around their website looking for photos showing rooms rather than individual paintings. The National Gallery in London for instance has rooms with pink-red patterned wallpaper, green, and deep orange.
White can be very stark; grey can easily be cold. Do you have a colour dominance in your paintings, or a small bit of something in quite a few, such as a yellow highlight or something? You might then use a version of this, or its complementary? Perhaps a touch of it into a grey?