Allowing colors to mix wet in wet on the paper produces wonderful transitions in color that cannot easily be achieved by mixing pigments on a palette.
My palette consisted only of yellow, red and blue. The variations of purples and greens came from the pigments mixing on the paper with each other as well as with the black ink that I used for the initial drawing. I find when I use only enough water to allow a slow, creeping flow, the colors still stay pure in large areas.
Sketch drawn first with black ink using dip pen, followed by wet-in-wet watercolor.