I haven't really tried this kind of colouring before so I have been experimenting,
I used Crafter's Companion Watercolour card, which is perfect for these pens. It's so thick I was able to colour on both sides.
You can colour straight onto the dry paper with your pens and dilute with a wet brush. I also tried colouring onto damp paper and then brushing out, again with a damp brush.
|Marine + Boulder|
= Black (for boot)
|Kingfisher blue + amber|
= grey (fish)
I was able to make plenty of shades for my picture. You can create new colours by scribbling onto a glass mat and using your brush to mix.
And here's the finished project: