Magpul posted a video last year showing how to dye their Sand PMAG which Nathan S. posted. GunMag Warehouse shows step by step how to do this without using a campfire and wilderness.
The process is rather simple but there are some things to know that others have not mentioned. Rit Dye is safe for use in the home but make sure you are mindful of what you put the dye in. If you use a pot to dip your PMAG in Rit Dye and hot water, make sure you clean the pot thoroughly. Back when I dabbled in dying ABS plastic with Rit Dye I was instructed to clean the pot with bleach. But Rit Dye instructions say you can use any regular house hold cleanser like Pine Sol, 409, etc. Dish detergent alone is not enough. I like GunMag Warehouse’s recommendation of getting cheap tin pans however if you plan on doing a lot of dying then just go to a Goodwill or Salvation Army and buy a cheap used pot as your dedicated dyeing receptacle.
The other issue to note is the color of the Rit Dye is not necessarily what you get when you dye the PMAG.
Here are the examples posted in the GunMag Warehouse article.
I am digging this Banana 40rd PMAG.