Well, I just don't let them dry in-between colours, and then things turn brown.
Like this first card, I wanted more 'blue sky' effect, maybe a hint of red as a sunset, but I put both colours in at the same time, and then went over with a brush... oh well... it turned reddish brown. But the green part actually turned out really neat, almost like a gate to a different world. The stamp is from Enjoy Life stamp set, designed by a fellow Stampin'Up! demonstrator Connie Fitzgerald as part of her Million Sales Achiever. Quite an achievement, I must say. And I'm also very happy to say, that I don't see myself reaching that level, so I don't have to worry about designing a stamp set (although I'm pretty sure Stampin'Up! would have someone help me...)
The greeting is cut out of Brusho Crystal coloured paper using Happy Birthday Framelit.
The second card uses the same stamp set 'Enjoy Life' and the background is also made using Brusho Crystals, but this time I kept the colours a bit more separated, and went over sideways with a paintbrush.
Both cards were made using Shimmery White card stock, which can handle watercoloring better than regular Whisper White card stock. I still haven't been able to keep my card bases straight... maybe I need to dry them with heat gun....