Could make up be put on scales?

Your fish folk got slime. It is easy to turn it pigmented.

Atop their scales the fish make slime. The slime keeps them from drying out.

https://www.thesprucepets.com/fish-slime-coat-1379911

What Is Slime Coat? The slime coat in fish is composed of a glycoprotein (protein with attached carbohydrate) that serves as the frontline barrier to virtually everything from large physical objects to tiny bacteria. This barrier also works to keep essential fluids and electrolytes in the fish, and helps the fish glide through the water by reducing surface resistance.

Much like humans have various layers of skin, fish have multiple skin layers as well. In their case, they have skin (dermis) that produces a layer of scales. The scales, in turn, are covered by a thin layer of epidermis. Goblet cells in the epidermis produce slime. Any break in the slime coat is similar to an abrasion on our outermost layer of skin. Losing a large portion of their slime coat would be like damaging a large portion of our skin.

The fish folk have developed many interesting ways to alter their appearance by adding substances to the slime or dissolving things into it. Pigment molecules dissolved into the slime persist for a long time, because the glycoprotein molecular skeleton persists a long time, especially in these landgoing fish folk that do not have the water washing it off all the time. S As the slime dries out they regenerate it from internal water reserves, much as we do with our eyes and mucous membranes. Pigment in the slime thus can persist. Sharp pigment boundaries might get blurry from diffusion after a few hours.

The fish can wash off old dirty slime (it does collect dirt) or old pigmented slime and regenerate a new coat. Perhaps they see this as normal hygienic maintenance.