You can paint a Night Sky Panel and get everyone to add stars and comets. Then let people choose the color paint they want to use to fill in each section of the design, leaving the black marker lines visible. You can go over the black marker lines with black paint if you like. These would be great to use for marking the Quarters for ritual circles. Whatever you decide, be prepared to accept any change in direction the Panel may take as it takes on a life of its own.
Art is an organic, fluid process .
Trust the Goddess. These can be goddess shapes, leaves, animals, stars, or whatever. If you plan on using these you will want to design the Panel's artwork before nailing the pieces to the Panel. Or for Panels that will be painted at Festivals or Gathering, just place the cutouts on the panel in a random fashion and trust that what should happen will happen. If you like you can spackle around the edges of the cutouts.