Another technique is a Radial (Draw) or Circular (Xara) Fountain Fill. Begin with light gray or white and end with the sky color. If you’re working in Draw add an Edge Pad of about 10 to force the sky color into the cloud. The brightest part of the cloud should be near the top. In Xara you can move the center of the fill by dragging its center. In Draw do this in the Fountain Fill menu by dragging the center of the Fill Proxy image.