Jep, Jenda you buy 40 volume peroxide and bleach powder (Jerome Russell, Superdrug or whatever) and then choose your highlight "colour". Therefore it really doesn't matter if your hair is already dyed or what your base colour is, there is no danger of an orange disaster.
Leave the bleach on until its yellow, rinse, dry, apply your "colour" (so if you want caramel highlights you'd probably choose a dark golden blonde or beige blonde, if you want really light ashy blonde then you'd get a light ash blonde, etc etc) and leave that for the developing time. Then rinse, make sure you like the colour, then you can remove the whole cap and give your hair a wash, though strictly speaking this isn't necessary, I like to give it all a good wash and condition.
Never, ever take the cap off until you are happy with the colour.
Its that simple!
The only problem with the kits you paint on is that sometimes they won't lift your hair up to the desired colour so you end up with bright orange highlights. There was someone on here the other day with that problem. Its not the end of the world but it means you have to colour over them, or live with them.