Below are before and after pictures of some M1900 grips that I cleaned. It was an easy decision as the grips seemed 'too dark', left grip had the safety chip and the pistol was a shooter.
Description of process in this thread:
http://forum.lugerforum.com/showthread.php?t=34747
It's now been over a year since I cleaned these, and some others. What I have found is after cleaning, the grips slowly darken a bit as the months go on. Likely, darker material below the surface wicks up to the top over time.