You're actualy using 5 colors. I suggest you turn the harsh white into a softer highlight, of purple tone, then use the blue sparingly on the face for the very brightest parts. I don't think you should worry about dithering the highlioght to the background, but instead the darker colors into the lighter, if you're going to dither at all. So far the dithering has only made it look fuzzy, and it might create an undesired texture when you finish.