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Your shampoo for one. The color you use is also a factor. Red dyes fade faster. I have been dying my own dirty blond hair to strawberry blond for over a decade so I know how fast the red fades...

Loreal.com has color matching. You can pick the color closest to your roots and then they will tell you what color to use. Colorspa is the best, its not harsh, its lasts a few months, and it fades...