Why does the paint option not work properly with elderly sims pets?

I have made my cat elderly (as my real cat is) and then painted over her as she isn't as grey as the elderly cat displayed. once I have completed this and backed out of paint to make the rest, she keeps retorting to the elderly colour

I have attempted to screenshot this and show what I mean. I know with human sims, this happens and you can just change their hair colour etc. The pets seems to allow you to paint over, but the overlay never stays. Is this a bug and it will fix?