I've used them before, the result you get depends what's underneath.
If you have dyed your own hair darker then you will get back to something approaching your own colour although it may take several attempts.
If you have lightened your hair with bleach you will get back to the bleach colour. It can only remove colour particles you have added to your hair, it can't replace colour particles that you have removed (by bleaching)
Also it smells like bin juice so prepare for that!