Copper colour change?

yes they can colour straight over this it would be considered a full head tint as to make it appear more natural they need to use more colours to blend in better i imagine it would come out looking very good if it is just a hslf hesd of highlights you are after then this would come out as a more streaky and unatural look but it is what you want that matters it is all about blending it in with your existing colour for a great effect