What is the purple ink used to stamp meat usually made from?

Grape skins.

Its a synthetic called C2 - Methyl Violet.

Hmmm... Depending on the country, it's either C2 Methyl Violet OR grape skins OR a secret USDA-approved formula!
The simple, traditional answer to this trivia question is that they make it from grape skins. For example, see http://www.corsinet.com/trivia/v-triv.html
But I just looked it up on the internet, and in Europe they do use the synthetic C2 dye. In the USA, it's a bit more of a mystery, though; the official government answer is that it "is made from a food-grade vegetable dye. The exact formula is proprietary/owned by the maker of the dye" according to the USDA.

https://www.clovermeadowsbeef.com/meat-inspection/

So I guess the answer partly depends on which country the meat was stamped in. :)