Yes ! One color much better , unless if u select one of the walls to be diffrent as an accent and make it more valuable.

Accent colors as well?

Yes , possible, and should be wonderful accenting with a decorative paint

Generally speaking, yes…but it is not as simple as it sounds…what color/colors you choose should be based on how much natural light…how high the ceilings are ..what type of decor you are planning..the flooring….the fall back is white, but there are many shades of white as well