Dying PMAGs is something of a national pastime. Even before Magpul rolled out their Sand polymer that takes dye exceptionally well, people were using Rit Dye on FDE and other colored PMAGS. There are long running and extensive posts on gun forums that pre-date the Sand polymer by a number of years. Now, with the availability of the PMAG Gen M3 Sand magazines, the popularity of dying these mags has only grown.
GunMag Warehouse recently posted what they call The Ultimate Rit Dye Guide for Dyeing your Magpul PMAG. The post shows the results from 25 different Rit Dye colors along with comments on some of the colors. It may also be worth your time to peruse the comments as there is some good information to be gleaned about the process of dying.