The exact method for color correction depends on the issue, however it usually involves adding the opposite color to cancel out unwanted tones. Color correction can be done with hair dye, food coloring, shampoo and even household items.
If the hair is too orange, tone it down with a blue-based blond hair color. Select a shade that is cool rather than warm, and apply it to wet hair. Applying the color to wet hair keeps it from covering the red completely. Check the color every few minutes before rinsing out.
If the problem is brassy tones in blond hair, make a toning shampoo out of food coloring. Add a pea size drop of blue food coloring to shampoo. Leave the shampoo in for a few minutes, and then follow up with your regular shampoo and conditioner afterwards. Do this daily until the blond is toned.
For hair that is too dark or looks uneven, use a shampoo made to clarify the hair. Wash it several times in a row, and follow up with a moisturizing conditioner. For really dark or uneven hair, mix equal parts shampoo, bleach powder, processor and water. Apply to the hair, leave in for five minutes, and shampoo out.
If the problem is that chlorine in a pool turned your hair green, apply ketchup to the hair. Red is the color opposite of green, so the red in the ketchup tones down the green. Likewise, the acid in the tomatoes helps lift the minerals from the hair.