Open a new layer above the sky's layer. Then select the Elliptical Marquee Tool and press the shift button to create a round marking. Paint it out with the necessary color.



Insert the same way several more circles:



Then we have to outline the elements, choosing previously the next option: Layer ? Layer Style ? Stroke.



Create a new layer above the circles' layer and group out both layers: Layer ? Create Clipping Mask (ctrl+g). The Gradient Tool will be helpful in decolorizing several of the circles.



Merge down the layers: Layer ? Merge Down (ctrl+e). On a new layer we have to represent one more set of the circles, changing a little the colors and moving the center. Place this layer lower than the big circle's layer: