I miss a color brush, a round brush with a edge decay option and "density-alpha" option to control how it merges with base color.
User would select a color, in a similar UI as "colorize" and paint over existing colors, this would be super handly to add some color variations over more widely colored areas. May be use Colorize or Intensity+ Colorize to add a base color and with the brush add the color variations where needed to add that artistic touch.