help?

I need some advice.

I had my naturally dark blonde hair dark brown for a long time. I needed to dye it again and I decided to go for a light warm brown, pretty close to my natural color. I used Loreal Preference, one that's specifically meant to lighten dark hair. It worked quite well on most of my hair...the trouble is that I had dark blonde roots showing, which were then lightened even further. So most of my hair is the colour I want, but my roots are platinum blonde and it looks awful.

I really don't want to resort to going to a salon, because I'm a starving college kid. Does anyone have any ideas as to how I should go about fixing this?

Thanks in advance.