Acrylic paints will be the easiest to use. The tough part is trying to apply a finish without taking weeks to get it done. You will be working with a glaze/paint mixture that can be adjusted for dramatic or subtle effects.
A color wash over the existing wall paint would work and it could be applied with a roller then the excess removed with rags. This would be easy to do although labor intensive.
Another possibility is using special roller covers designed to apply a color wash. The overall look isn't as good but it's less labor intensive.