Hello!
When i attached to background(environment section) a ramp texture with three different colors and i tried to render a sphere with mirror material on it, so it reflect only one color. it is bug?
Could you do : ColorBalance: colorGain, colorOffset - in the FIle node. it will help to increase brightness or darknes of .exr files.
In general I really like this rendering,
You are doing a very wonderful job.
Thank you!