I'm in the process of getting rid of blonde hair and have colored it a nice medium brown/auburn color (Preference by L'Oreal). After about 3 weeks, the old blonde starts showing through and I don't like the way it looks. Is it safe to re-color after 3 weeks while I'm growing my hair out, or do I need to wait longer?

It is recommended that you color your hair every 4 to 6 weeks. But, if your hair is in good condition, you can re-color it right away.