Yellow HairQ: I'm naturally a brunette and decided to go blonde, but now my hair looks yellow. What went wrong?
A: Actually, it's possible that nothing went 'wrong', it sounds like your hair has a high concentration of yellow as a contributing pigment, and if you used a haircolor after lightening it, the color probably had a gold base. The yellow can be corrected, though.
My first recommendation is of course that you see your stylist. He or she can mix together a corrective color formula to minimize the yellow color, probably using a light (level 10) violet-based color with 10 volume peroxide mixed and combined with equal parts of a moisture-rich conditioner. The violet color base, since it is opposite yellow on the color chart, will tone down the yellow.
There are also products available (shampoos and conditioners) that are designed to balance out yellowing in blonde hair. They have formulations using violet based tint. One such product is found in Clairol's 'Shimmer Lights' line. They have a formula specifically for light blonde hair.
Basic hair coloring information
Hair colors and our color palette
Things that can go wrong when coloring hair