Not much is going to stick to that kind of rubber without irreparably damaging the rubber...that's why it's pigmented before manufacture. You can 'stain' it, though. The mold stains prove that. Maybe draw some designs in it with permanent markers, like Sharpies. Experiment until you find something that really stains it. You might still have a lot of orange, but stripes and swirls and dots could be really interesting! Clean the fins with rubbing alcohol before starting. Alcohol will remove permanent marker from lots of surfaces, so if you draw on it and wipe it off with alcohol, and the stain remains, you've found your coloring solution!