Literal translation: One who mixes with vermilion will turn red, one who touches ink shall become black.
There is a Chinese proverb warning us that who we come into contact with will have influence on us. In the saying, red is used to represent desirable transformation, while to become black is bad.
Whitty obviously has been spending too much time with Blackie. She is going through her first serious moulting which replaces her baby coat with an adult one. We are joking that the petshop must have dyed her fur white while she was on sale, and now we are seeing her true colours. See the distinct black lines forming in her new coat, and the ears and forehead look dusty.