Ok. this can usually be fixed easily.
Semi-perminant colour is designed to wash out over time.
The best way to remove or lighten this coluor is to buy a shampoo for oily hair.
Use the shampoo to wash your hair, try shampooing the hair three times in the shower. After the third time use normal shower gel on the hair. Rinse it very well and let it dry naturally.
As the hair drys naturally you then can apply some lemon juice to the hair, combing it well into the hair and all they way to the end.
Leave this on for about 1/2 hour, then rinse it out.
Then apply a good conditioner to your hair.
This will definately speed up the lightening process.
If you want a very quick soultion you can have some natural highlights in the hair to break up the dark colour.
If you cant do this or afford it just try the above method.
Hope this helps you!