I am a blonde and just had my hair colored professionally to add depth. I have more dark (dark blonde) that to me looks more mousey brown than I would like. I was told I could use "laundry" detergent and apply for 3 minutes to lighten the hue. Is this OK and how long should I leave it on? Should I follow up with anything specific for conditioning? My hair is very fine and therefore gets oily frequently. Thanks.

Sweetie, I would recommend that you go back to the salon and have the hairstylist correct the color if you're unsatisfied with the results. Most salons don't want a bad reputation, so they will fix their clients hair free of charge.