All you need for a color romp:
Watercolor set (kid’s sets are fine), your journal (I'm using a watercolor Moleskine) a spray mister, watercolor brush, rag and cup of water.
Wet the colors down with the mister to moisten. Start painting with the lightest value colors (yellows and greens) and move around the page. Keep your colors moist but not dripping wet so you’ll get a full saturation.
As the colors dry, layer a second color on top of the first color. Try blue over yellow and red and red over orange. Try laying purple over yellow. Mud? Or is it browns and grays?
Make little notes to yourself as you go along. Tab your page for future reference!
For more about watercolors, see my Free Online Watercolor Primer