I have dark brown hair, that I have to do the grey roots every two weeks, I am considering having highlights put in at the salon, but I am worried about upkeep at home, if I touch up the roots, will all my hair grab the colour and make the highlight a waste of money, I'm being offered the highlights at a trainee price and couldn't afford to go to the salon every fortnight to have the roots done?

Highlights are simply parts of your hair that's dyed or bleached. .

If your hair grows fast enough that you need to touch up your roots every two weeks, the highlights would be grown out at the root by then anyway. In other words, you wouldn't be covering the highlights, because that portion would have already grown out.