I  have been experimenting with iridescent paints and metallic markers. If you paint with watercolors after having marked paper with a metallic Sharpie, the Sharpie will resist the watercolor paint because the sharpie is oil based.You may have to remark the area after you are done paining but nonetheless, it will shimmer.  The green dress below was Veridian Green mixed with an iridescent medium. It created a beautiful stary night effect.