An image rendered with GI (and low ray depth) is very bright in some areas and very dark in others. A more balanced allocation of color and brightness can help make the lighting more consistent.
One solution is to edit the internal rendering’s interpretation via Color Mapping.
Although these settings look inconspicuous, you should take the time to try them all out.
As a rule:
•Exponential and HSV Model should be left active.
•Use the Multipliers to fine-tune your image.
Exponential
The color dispersion will be controlled exponentially, not linearly.
HSV Model
If this option is active, the HSV (Hue, Saturation, Value) model’s colors will be converted and only the V (brightness) component will be affected. The colors will remain identical, e.g. a dark blue will not be turned into a purple.
Affect Background
Here you can define whether or not Color Mapping should affect the background (if you are using one - such as Sky or Background.)
Dark Multiplier/Bright Multiplier
These settings let you strengthen or weaken dark and light colors.