Squirrels' teeth are normally brown. Most rodents have an orangey layer on their teeth, but I'm not sure why.
Anyways. I'm going to say food coloring of some sort. Possibly drink mix.
Here's the story I'm basing it on: When I was a kid, I had a bunch of fancy mice (and some other rodents) for pets, and I started breeding them as a hobby.
Some of them were albino - PEWs - and hard to tell apart. And one of the males was VERY aggressive. He'd slaughter any other male in his cage. And I didn't have the heart to kill him, so I named him 'Mousolini' and kept him in his own cage.
To tell him apart from the other PEWs, I put one drop of blue food coloring on his head, thinking it would dye that spot on his fur. Instead, he groomed himself and ate the food coloring, and his entire body turned light blue, his skin turned purple, and though it faded over time, he was blue for the rest of his life. (also his poops were really pretty for a few days - they looked like turquoise beads. lol)