oh wise and terrible internets, lend me your advice

As you know, I've recently done a blue/purple/green dye-job on my hair. It's awesome, and I'd like to keep it looking awesome as long as possible. I already (a). don't wash my hair but 2-3 times a week, in cool water, and (b). use a good salon-brand color-preserving shampoo/conditioner with which I am quite happy, so I don't need any suggestions on these points.

But I want to hear your opinions on dry shampoos. Powders, sprays, whatever. I'd like something to freshen up the ol' technicolor...