If the wall wasn't primed or the color was previously a flat paint, it is possible the paint is absorbing into the wall. Ask the paint store to tint a primer to match or get very close to the color of the paint and then paint with the primer. Go over it with another coat of the regular paint when you are finished. Also, look on line for painting instructions. I know painting sounds and looks easy, but it is amazingly easy to spread the paint too thin or to rework over the areas you've painted and actually pull some of the paint off of the wall.