Things They Should Invent: removable on demand hair dye

The problem with dyeing your hair is that you have to either keep re-dyeing, or grow it out and have trashy-looking roots. Yes, there are temporary dyes on the drugstore shelf, but they're unreliable - sometimes they wash out way too quickly, sometimes they don't wash out at all. I was once walking around with six-inch roots from what was supposed to be a 24-day hair dye, which you can get away with when you're a teenager but doesn't work so well when you're pushing 30. I'd rather like some interesting colour, but it's an expensive commitment to do and maintain to a standard that's suitable for my age and hair length.

So here's what I want: hair dye that is permanent if left to its own devices, but can be washed out with a specific shampoo designed for that express purpose. Regular shampoo won't budge it and if you do it once without maintenance you'll get roots like usual. But then, once the roots are too much, if you don't want to re-dye, you can buy this special shampoo and wash the colour right out so you're back to your natural colour.