It'll depend on used renderer / material. Please feel free to share more details regarding your scene set-up.
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. Please feel free to "optimize" repeatedly to try different settings at render.
Hope that helps.