You can put colors on the stamp and then spritz with water before you stamp; repeat until you get the look you want. You can also lay down color on a clear block using a Stampin’ Write Marker, mist with water and then stamp. I always use Watercolor Paper when I’m coloring with water; I think the results are much better than using regular cardstock.
One more way to do a watercolor wash is to load up an Aqua Painter with the color you want and then move the AP along on the paper, changing up colors for an “ombre” look, like in this card.
Here’s a short video showing another way to do a Watercolor Wash, using heat embossing…enjoy!