When I was quite young (around 20), I took my aunt's advice and tried dyeing my hair to even out the colour (it was getting light at the ends from sun bleaching. I wanted all of my hair to be its usual medium reddish brown.)
BIG mistake. I discovered that my hair was VERY absorbant and that most brown and reddish-brown dyes contain purple pigment that is supposed to rinse out. In my case it did not and my hair turned a glorious aubergine colour. Needless to say, it was not the colour I wanted so I went to the local store and got a hair dye remover.
At the time, I didn't know that ALL permanent dyes, regardless of the colour you are trying to achieve, bleach your natural colour out so that it won't "interfere" with their colour. So after removing the dye, I was left with hair the colour of flame. And it continued to lighten over the next couple of months to light strawberry blonde. And I look HORRIBLE as a blonde.
Happily, my hair grows about three times faster than average, so after a couple of months, when the dark roots were three inches long, I had it cut and was back to being a brunette! I certainly did my happy dance that day...