You probably heard of BLEACH LONDON, if not check out this post I did a couple years ago about their dip dye that started a hair revolution. BLEACH have joined with Boots for a full hair colour range that's seriously fab. I haven't tested them on myself because I'm super old, but I've seen it on others and it looks brill.
If you're unsure about permanently dying your hair rainbow colours I completely understand, as do Boots and Bleach, which is why they teamed up to create really easy to use non-permanent hair colour creams in Bleach's 12 signature bubblegum colours. They are very easy to use and fade with 2-10 washes, depending on tone of hair. They also sell a "Washing Out Liquid" Colour Reducing Shampoo (£5) which helps speed up the fading process when you fancy a change of colour or you want to get rid of stubborn pigments. These are such a fantastic and cheap way to a get the cool BLEACH colour look, and if you do it definitely check out BLEACH's blog for hair colour inspo.