If it's a warm colour it should be ok, does it match your roots in shade (ie light/dark not cool/warm)?
If it does it should take, and cover the brassiness, and it'll be more vibrant on the highlights. The previously highlighted bits might fade quicker though as you've stripped the pigment. Try getting hold of a good protein treatment first THEN moisturise! Protein makes your hair stronger, but you must moisturise it afterwards to balance out the strength with softness and prevent crispiness
Anyway... a protein treatment "fills out" the highlighted hair shafts so the colour can "take" better. I love joico kpac reconstructor, it's super expensive and I can't afford big bottles in the salon, so check out amazon to find little bottles and maybe sachets. Aussie 3 min miracle is awesome as a post-protein moisturiser, it's quite cheap and they sell little sachets for 99p :-)
(edit: check seller's feedback and especially the product reviews on amazon if you use it as there's some fakes floating around and you want to make sure you're getting the real deal).
If there's just too big a colour difference between the roots and ends, try a light warm brown first as a filler colour then the mahogany
Sorry for long explanation but I LOVE hair colouring, I mostly do my own (been blonde, silver, red, pink, purple, platinum, brunette, black, do foils, ombré, pretty much the lot!) and I could talk your ear off for hours and bore you senseless about hair ;-)