Watercolor as a medium is ripe for experimentation. Brushed onto paper you have soaked in water, it will spread, creating fuzzy designs (not too wet or it will simply fade away!) Brushed on dry it will skip over the textures of the paper and leave a trail of color with sparkly white showing through. A strong wash sprinkled with salt as it is drying will turn into a splash of stars (hint . . . use indigo or a deep blue and salt and you do get stars!) One color dropped into another while both are still wet will lead to "charging" where one color pushes the other out of the way in interesting patterns.
Here is an example of a watercolor I did using salt to make stars (actually it is more like frost!): http://i56.photobucket.com/albums/g172/lefthandedly/100_2126.jpg
If you are not familiar with some of the fun you can have, check this out: http://www.watercolorpainting.com/watercolor-tutorials.htm
Here is what you can do with masking fluid splattered on the paper before you begin painting, and you simply rub off the rubber-cement like fluid after it, and the artwork, is dry. Masking can be painted on or splattered, like I did here, or you can even mask with tape, removing it after the paint has dried: http://i56.photobucket.com/albums/g172/lefthandedly/100_0123_2.jpg
A neat trick to a clean and attractive finished painting is to tape it down to a board with painter's quick release tape, leaving at least 1" all around. When you finish and the painting is dry, removing the tape will reveal a clean white border!
Another little trick is to test colors on a scrap of paper before laying them down on your painting . . . this can help you avoid making "mud," which happens when too many colors, or colors from the wrong groups, are mixed together. I have done many paintings using only the three primary colors and mixing all the others myself: the only caution is to be sure you are not mixing warm and cool hues that will turn muddy. I use permanent rose, new gamboge and antwerp blue for primary color studies like this one: http://i56.photobucket.com/albums/g172/lefthandedly/100_2122.jpg
You could have lots of fun with a celestial theme . . . and with a little experimentation, you could discover a new planet!