I have a wound 5th course string, so maybe this is a little off-topic :). Anyhow, my wound strings are darker by the rose where the right thumb touches them. But they are lighter close to the 3rd and 4th frets, where the left-hand fingers touch them.
Thumb and fingers get washed about the same, so I have no good hypothesis for this discrepancy. I was careful not to incorporate effects of lighting angles into my perceptions. To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
