Walkthrough

1. How to Get All Paints
2. Speak to Smearguru for the first time on the northern part of Bleak Beach's map. Give him a Leppa Berry, and he'll request for you to create a dye using Leppa Berries.
3. From Smearguru's hint, find a Pokemon with the Crush specialty and have them process Leppa Berries into dye. The easiest one to access is Onix.
4. After having a Leppa Berry crushed into powder, present it to Smearguru, who will then decide to use it as paint. This will unlock the option to speak to Smearguru about changing the colors of certain furniture!

Walkthrough source: Game8.