As some of you might know - a few years ago I picked up Rebelle3 (and upgraded recently to Rebelle4) and I think it's the best thing on the market for digital watercolours as it does a great job of simulating watercolour pigment mixing (although Rebelle3 uses the RGB method rather than a more 'natural' colour mixing). If you've seen my old Kickstarter - that first image of Horos/Penelope was made entirely in Rebelle3! In fact, let me post it, and some other things I've done in Rebelle!
I'm honestly surprised (but glad) to see it in a Humble Bundle so I encourage you, if even curious about it, to maybe check it out? I'm not getting any kickbacks for this, I just really love to play around in Rebelle!
And there is more than just watercolours but honestly I bought it entirely for the watercolour-like features!