Hi Lamka
I like your idea ist colorful and sounds fun.
Here is one painting skill from my sketchbook:
"Palette of Death":
If your skill isnt a phantasy and your paintings dont arise by copy and paste. You might become the Master of Arts by unleashing a deadly-chromatic Storm. By using this skill your castbar switch in "palette-mode" witch provides up to 10 colors and now by the Divinity of Fortune (or some programmed random-generator) colors are presented to you and you have to react fast (getting faster) and click right button.
Each correct click gives one stack (can do xx damage) till you fail or release your Opus you collecting the stacks.
If released you deal the damage as promised and if failed you get xx% minus the promised damage but also triggered.
Could work on single target as well.
Roy