Dyed my hair, roots are too light... help!?

Sorry for the long explanation here, but bear with me. Last summer I decided to go blonde, and after a few months of it, I decided it wasn't a good look on me, and opted to go back to something close to my natural colour, a medium brown. I kept it for a while, but it started fading out, and the tone difference between the dye job and my roots was easy to see. So I decided to get a hair cut to remove any damaged hair and decided to dye it again, this time something a bit darker, so hopefully it wouldn't fade out too much. Well, I guess I made a mistake there. I chose 'Dark Mahogany Brown' by L'oreal, Colour number CB45. I like the colour, it's a bit darker than I had hoped, but it had some nice red tones in it. Only problem is, my roots didn't seem to absorb as much of the colour, because they look almost a coppery, and it's extremely lighter than the rest of my hair. I wanted to scream. I don't know how to fix it, but it needs to be done fast, I have a big event coming up in 2 days, and don't have time for a salon, trust me, I called, they're booked up. Anyone have any ideas of what might fix this? Should I try to re-dye just my roots with the same colour, or maybe re-dye my roots with a slightly darker colour, but the same tone? Ugh. HELP!