So, I have already baked my vanilla cakes and client just sent a message to see if there is some way I can make the inside of the cake pink and purple. Normally I would have coloured the batter prior to baking, but that isn't an option now. I told her I could use pink and purple buttercream between the layers, but just thought I would post the question to see if anyone had ever coloured a baked cake?
No, not after it's baked. Anything you'd use would make the cake disgustingly soggy.
She asked too late, so it's her problem.
If you have the time--and want--to re-bake it, it's up to you. You should tell her that there's an upcharge for it, though, too.
AThe only thing you can do would be to stick a toothpick in the cakes several times and then drizzle pink and purple jello on the cake. Then chill them. It may make the cakes soggy if you aren't careful but it's the only thing I can think of.
Nothing that wouldn't compromise the texture, I'd just go with coloured buttercream.
This^^ or maybe you could make a simple syrup, color it and drizzle each layer in the appropriate color. I know a lot of pastry chefs like Joshua John Russell, use simple syrups for extra flavor and moisture anyway.