I agree with the others. If called that, (Dirty Red/Red) I don't think it would have the same connotation as being called "Cat." Though I have only heard of being cat from various forum boards, it wasn't anything that sounded flattering.
Is it a term of endearment? Since they are the "dirty, nasty Q-dogs"??? I don't know.
If you (those who have been called those names) are unsure of their meaning, (whether they are meant to disrespect you, or what ever) why don't you ask them about it?