Do you remember when all pistachios you could buy were a bright red? At least all of the ones sold in the shell? It wasn't a particular hybrid of the flavorful little nut, the red was food coloring. And a feeding frenzy of pulling the nuts apart to get at the greenish nugget inside would leave your fingers stained.
So, why aren't they red anymore, and why were they red in the first place? This article from Huffington Post explains it all for you.