Always heard that the brown grips were mainly due to sun exposure. I know that I have seen (and owned) a few examples that had one side brown and the other black....... Perhaps from laying on one side in a display case or on a shelf somewhere? They did make the two tone mottled brown/black grips at one time so maybe there were some all brown versions. Don't think I would lay a grip on the roof to find out if it changed over time, a windowsill would seem to be a better choice.