Between manual gathering and Crush specialists, keeping Yellow Paint stocked isn't hard. The Crush route is less effort once you've assigned the right Pokemon to the job.

Paint used to color items yellow

Raw Material
Aspear BerryCrushCrushYellow Paint
ShopSmearguru / Palette Town Shop
Bleak Beach (Smearguru hut), Palette Town (Level 5)
LarvitarCrushConkeldurrCrushGravelerCrushShieldonCrushGeodudeCrushPawmoCrushDugtrioCrushTyrantrumCrush

You can craft Paint Balloon Lime, Paint Balloon Orange and 1 more using Yellow Paint. These 3 recipes are mostly Other items that help fill out your island.

3 recipes
Paint Balloon LimeOther
Green Paintx1›Yellow Paintx1
Paint Balloon OrangeOther
Red Paintx1›Yellow Paintx1
Paint Balloon YellowOther
Yellow Paintx2