Why do the hairdressers have to bleach my hair?

I've been semi permanent dying my hair to a dark brown (natural hair colour=medium to dark brown) for a while now to cover up my old highlights which were yellow and caramel, which i decided after a while i didn't like. Now i've stopped, after two months semi permanent dying my hair and decided i want some lighter, subtle, browner highlights put in my hair. The hairdresser said i have to bleach it first. How long does the bleach last? What will it do to my hair? Will it go orange/yellow :( ? I'm scared. They said not to worry about the condition, they'd do plenty of treatments. I'm still worried though cos i've never bleached in my life! any advice please?!Why do the hairdressers have to bleach my hair?
colour will NOT lighten colour which is why she has to bleach your hair lighter in order for your hair to go lighter. bleach is permanent, it has to grow out, bleaching is stripping the natural pigment from your hair so what has been stripped cannot be washed out. no, unlikely that your hair will be orange if she is just re-colouring over the bleached strands to a light brown shade.Why do the hairdressers have to bleach my hair?
They HAVE to do it because you can't dye hair lighter ;)