You can normally lighten colour by applying creme peroxide...that`s the `catalyst` which is mixed with the colour-paste ---- obviously I`m unaware of how / what product/s you used to darken the hair, but I use a tube of colour `paste` and mix it with creme peroxide...the less peroxide used-the deeper the colour, and the more peroxide mixed-in, the lighter the colour will be [ the duration it`s left on the hair will also affect how `deep`it goes] I have never had this issue simply because I never allowed the `mix`on the hair for longer than a few minutes.... I asked the salon person [who I was dealing with at the time] what I should do if I went too deep with the colour and she said to just apply the peroxide to the hair to lighten the colour again.... I`m unable to give you a `time`on it...just try it for 3 minutes first and see how it goes [ try a very small/unnoticeable section of hair first until you see how it looks] Creme peroxide is a white, slightly ` gooey` liquid/substance. Here`s one example of what I`m referring-to: http://www.ebay.ie/itm/WELLA-WELLOXON-PE...0615751025
and here is an item I found too --I`d advise AGAINST mixing bleach with the creme peroxide.... just try the peroxide on it`s own and keep a close eye on it... I cannot give you any definite timings.....just proceed cautiously! http://www.ehow.com/how_5061121_remove-h...loper.html