A few other things to remember when embarking on a major colour change:

If you are going to bleach your hair, don't wash it for at least four days first. Longer is better. Let it get skanky and greasy. People will not sit next to you on the bus, but, your scalp will not sizzle and burn, and the chances of blistered skin are reduced.

If you have very dark hair, it will take bloody ages to bleach down. You will almost certainly look like hell when it hits that light yellow tone.

It's worth the money to get at least the first bleach job done by a decent hairdresser. It's cheaper than a wig.

If you are planning on a deliciously unnatural colour, like blue or purple, you really do have to bleach it that pale first, even if you have naturally lightish hair.

Blue toned dyes, and others with plenty of blue in them come out of your hair very fast indeed. A couple of washes and you are looking faded and bedraggled. Dyes that are heavier on pink or red will last longer. But not by that much. Strangely, yellow based colours are the most persistent. And they will not fade and disolve evenly. Of course, with bleached hair, you don't want to wash it as often as you would have done with unabused hair.

Bleached hair loves conditioner. That miracle-fix-it-all-once-a-month stuff? Use it every time. Say farewell to brittleness and split ends.

Be prepared to walk around, dazzled by how your hair colour changes in different lights. Be prepared for little old ladies to scowl at you, and shop assistants to rush over and ooh and ahh, be prepared to have very long conversations in the pub about the qualities of different brands of hair dye. (my vote is for fudge colour box purple haze. Be prepared for purple smears on white pillow cases if you go to bed with damp hair.

After about a year, be prepared to see this strange colour as your natural shade.