A Lick of Paint is an achievement that requires the player to select a colour for a new kitten costing 100 coins.
Gertrude will not sell the player a new kitten if they already have a kitten or cat with them or in the bank. However, if the cat is in the Menagerie of the player-owned house, she will. Wear the ring of Charos so that she can be charmed, and select the colour.