This tends to happen a lot in ceilings in a model of a room for example. Sometimes I do the above mentioned way in combination with putting a distant light below the model, pointing directly upward and casting no shadows. Tends to lighten up these bottom side surfaces and not effect the rest of the rendering. Hope this helps a bit. The color is only part of it you also have to match the properties of the material to reflect light.
All paints reflect light different and effect how they look.
If we could educate our clients a bit about the nature of color it would help. Having spent many years in the architectural wooodwork business I can assure you color is a very illusive thing. If you were to actually paint the room or object in question it would most likley not match the paint sample. Certainally as viewed in some perspectives with various light sources it would not. There is no reason why a digital model of the scene would not also have simular properties, in fact, is that not the objective to present images with some degree of realisim.