Should i cover up highlights?

I had my darkish brown hair highlighted an ash blonde. It looks fine but it is growing out and I dont want to fix up the roots. I wondered (if i chose to) what would happen if i put a temp brown dye on it:

1) Would it wash out at all or would my highlights soak up the color too much?

2) I asked the girl who cut my hair recently about it and she said it would turn the blond orange...is this true? Doesn`t it depend on which color you use (a blue base or red)?

Thanks for any imput