Of course I can't say for certain, but there are N95 masks that are dyed and have designs on them. That said, those masks are likely tested with their design.

Just my 2 cents, but I'd probably say if you draw on a small portion of the mask, like whiskers etc, it would be fine. The mask would still filter and I doubt the dye from a marker would change its effectiveness. The worst thing I can imagine is it possibly causing slightly less airflow in the parts that are colored, but I even doubt that unless it was just overloaded.

I'd be interested in other peoples opinion too, kinda curious myself.