Hair dye help?

[sazzie]

My sister wanted to dye her hair a dark shade of blonde (she was dark brown before), so i stripped the colour and put highlights through it. It's a copper red/brown colour, with lighter hightlights. I was going to leave it a few weeks to get it in better condition, then go lighter gradually. BUT... she has a prom this month and wanted to dye it darker, but not permanently. So, i put a wash out dye over it, but it didnt take to the lighter parts. Why is this? And is there any dark wash out dye that will take to the highlights? I dont want to put a dark permanent dye on it as the lightening of the hair would have been a waste of time. Any advice?
Thanks in advance!