After bleaching my hair for years I want to go dark,can I do this in one step?:

ohsohappy

Hmm, this might be tricky. If you're planning on doing it at home, it might not work very well and it could fade out. I suggest that if you plan to dye it, go get it done professionally. You don't want it to end up looking like this.
(not my hair, I found it on the internet.)

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Perito

The formulators of dark dyes assume that there will be lots of red in the hair that they need to counteract. This is because the action of hydrogen peroxide on natural pigments leaves a lot of red in the hair. If you've bleached it, it's undoubtedly blonde and you'll have to replace the red. Some people try to add red dye to the brown that they want. This is tricky because it's hard to predict exactly how much red needs to be added. If you don't get enough, the hair will have a greenish cast (they put green in the dye to counteract the red -- green + red = brown).

Most of the time, you'll dye the hair red and then dye it brown. The "dye-it-red" process is called "filling the hair" and the dyes are called "fillers" (there are other types of fillers, so don't get confused). Demi-permanent haircolors (Castings, Radiance, Natural Instincts, etc.) are the preferred dyes for bleached hairs. They're marketed as "semi-permanent" but they're actually more permanent on bleached hair than "permanent" dyes. The reason has to do with the porosity of the hair and the size of the demi-perm molecule.

Fading is a big problem for people who do "tintbacks". You'll have to deal with it. The best way is to periodially dye only the bleached ends with a demi-permanents.