Oh, no, purple mud! And you know what else I just noticed? Dilbert doesn't have a mouth! MY DEMANDS FOR REAL-WORLD ACCURACY HAVE BEEN THWARTED FOR THE LAST TIME!
You must admit, if you didn't know then you'd think that was mud. Someone needs to coordinate whoever colours these. Ratbert is the other sticking point. Or perhaps he just uses a different fur dye everyday. Who knows?