First you need to buy the schematic of the paint from Sally.
How to Get Paint
Beach rocks that spawn every day give 4 chalk, 2 stone per rock.
Flowers, coal, chalk and either glue or the other stuff next to it in Moss’s shop that look powder tin.
Depending on which one you have it either fabric dye or paint for furniture or aisle once unlocked. It’s bit buggy though still.
There also in your inventory window a tab that shows quest, but also recipes too. Paints and fabric dye are made in crafting bench at Sally’s plus there is other stuff you can make like fabric, brick, stone slabs, etc.
The designer makes objects like furniture and beds, it has paint tab but for me it sometimes freezes. The aisle is unlocked when Sally’s friend moves in, you make your own floor tiles, wall paper, shirts.