M.R.
I would try lemon juice, diluted 1/2 and 1/2.
But it sounds rather pretty for the summer, so maybe you could just add a few extra chunks around your head to even it all out and then it will all wash out together? You could start a new highlighting trend..women would save bundles of time and money at the salons...
Honestly, I have some grey I'd like to cover up...how bad does it look?