You can't go wrong with blue. Since the colors in nebula art are always sort of arbitrary, why not go with the one that gives you the serene mood you want? This one is called "Star Wave," meant to suggest a cosmic ocean of gas and stars. There's no reason why the curling forms of ocean water wouldn't also occur in space, as long as there are similar forces creating them. This nebula is also hosting fresh new stars.
And meanwhile, a happy Winter Solstice to all my friends and readers in the Northern Hemisphere! The days are now getting longer.
"Star Wave," acrylic on illustration board, 12" x 8", January 1988.