Rabbit pee should generally be yellow to a reddish color. Changes in color often come from (but are certainly not limited too) dietary changes. Have you fed them anything new or different, or could they have gotten into anything? If it truly looks like chocolate milk, from what I understand, that’s not normal and a vet visit might be in order. However, I had an issue with my buns where their pee was a much darker color (more brown than red) and ended up just being some nasty, watery diarrhea that cleared up in a few days. Another thing to think about is bladder sludge. Make sure they’re drinking a ton of water. If nothing changes soon, see a vet. Good luck!!