Will ingesting food coloring harm kittens?

I foster cats and kittens from the Humane Society. Once in a while if I have kittens that are "twins" I like to be able to tell the difference between them. Sometimes I put hair bands around their necks (of course, not too tight), but if they are too small, sometimes I put a dot of food coloring on their paw. Will this harm them, or possibly, their mother, if they ingest it? Thank you, cjg. ; )