I last coloured it about 7-8wks ago, dark.... it's been dark for a while, but I think I want to go back to blonde.... when can I colour it lighter without it turning green or purple or something!!
as long as you wait 4 weeks between colouring you should be fine, make sure you leave it on to develop long enough!!!!!!!
mine went badly wrong once and I redied it the next day by using prelightener bleach and then a dye on top. probably really bad for my hair but it fixed it.