I love to use my Lyra watercolor crayons. They make it easy to make a quick watercolor card!
I colored the outside of the rings with various colors of watercolor crayons. I then smoothed the color with a waterbrush to dissolve the paint. Hint...go over the outside color with the brush and then clean of the brush before dragging the color towards the center. By getting rid of the extra paint, it is easier to make it look like there is a gradation of color from outside edge to the center of the circle.
When my card was dry, I put it under a heavy book so that the watercolor paper would become flat again.