1) Watercolour paper is not the only type of paper this works with. The Stampin' Up markers are streakless on kraft, natural white or ivory, and confetti white or cream. I find that using watercolour paper, well, it makes my project look watercoloured. These other papers I've listed don't have that effect on the appearance, plus they are less expensive! The crab below was stamped with black stazon on natural white paper.
2) Instead of outline colouring your main image and pulling the colour away with an aquapainter, try squiggling a bit of colour onto a piece of plastic. Blend that into the image if you want a more gradual lightening of colour.
3) Consider getting a light grey alcohol-based marker if you like the subtle outline shadowed look. You can find a 10% grey marker from Prismacolor for pretty cheap. It adds such a nice shadow to items.