I've been doodling around with art rage to see what I can come up with. So far - this is the result of a quick tinker:

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The watercolours work really well as a tint layer. There any white is read as transparent (it seems very close to multiply). Merging each layer down when it's done effectively 'dries' the watercolour layer and keeps the resource use in check. It's a lovely program and the watercolour paint dynamics are very very good.