Well, that'll depend mainly on used material (I would suggest to use a VRayMtl) and the illumination scheme.
For quick adjustments you might find useful the "RGB Level
" parameter in the Material Optimizer
. It's applied to all diffuse maps, except those excluded by name ("trunk", "bark" and so, if checked). The default value is 1.0, and setting 0.0 preserves the current level in the texture map. It's possible to "optimize" repeatedly to try different settings at render.
Hope that helps, but please feel free to post more details regarding your issue.