I'm all for a quick style change with zero commitment. I'm happy with my brown and blonde - but sometimes want to indulge and see what it's like to have a different look. The Fudge Urban Hair Chalks (£3.99) are a great, quick and easy way to add some colour. And you can personalise to suit.
All you do is wrap a towel round your shoulders, and rub the chalk straight onto dry hair. A quick spritz of hairspray will help the colour stay put. It can be bright, or if you fancy trying some pastel shades, rub the Iced White shade on first then the colour. I think the Iced White shade would help those with darker hair achieve a more noticeable result. One wash and the colour will be gone.
You can pick up a whole range of shades from Boots - where they are currently on offer, or online.