I have a cordate plant with reddish leaves.  Here is the observation.

This AM, a piece of driftwood "fell" across the leaves.  I did not think much
of it and resolved to "fix" it tonight.  When I did so, I noticed the reddish
color of the leaves was "bleached" in the parts of the leaves that were close
to where the driftwood lay.  

The plant otherwise seems fine, and I am sure it will "recover".

Was it the pH, the tannic acid, or less light (shading)?   It happened in
HOURS.  Has anyone else observed something similar?  Enquiring minds want to
know.   Dave

Dave Gomberg, Experimenta      San Francisco CA USA   gomberg at wcf_com