Go to my "My helmet progress pics" thread. and see my painting guide. Use the paint misting technique I described and spray on ligher shades spray can paint of your dome and cheecks tp lighten it up.
Or you can just get lighter paint and repaint it.
You may try white or gray pastel chalk to tone it down, I know black darkens it up when weathering, so lighter chalk may work as well.
My apint was too dark and also too shiny.I lightly sanded it with a fine grit sanding sponge and I couldn't believe how much it helped.It looks MUCH better now .It lightened it up ,dulled it down and left no sanding scratches.
I guess its true how those who dont learn from history are doomed to repeat it becuz i make my helmets light collered alot and it usally turns out bad, i lighted ym boab fett one a little bit too much and then it looked like someone who just got out of a bounty hunter make up and fashion show.