I always use Stampin' UP! Watercolour Crayons and my aqua painter.
I find that if I pick up the colour from the crayon with my aqua painter (rather than colouring directly on the image), I don't get streaks and I can better control the amount of colour I'm applying.
I just did an image this week, and you can see it on my blog here:
Handstamped from the Heart: Bewitching the Paper Craft Crew