Nothing But the Best

You have to give the painter medicine of the top quality tier, which means brewing the potion he needs to "Henry's quality" at an alchemy bench rather than buying or bodging a weak one. That needs a solid Alchemy level (around 16) and the right ingredients, plus brewing it carefully (optimal steps, right number of stirs/boils). Skill up your Alchemy first, gather the ingredients, and hand over the best version. Missable - a lesser potion won't count and the window closes.