When coloring hair, sometimes the color does not take evenly from root to end. Other times, uneven hair color results from uneven fading. Both situations have probably occurred because your hair is damaged -- most likely dry. Dry ends do not take color the same and do not hold it as well. Dealing with uneven color is really dealing with damaged hair. The best course is heavy conditioning. But for the short-term fix, correcting the color now, you have to either lighten the hair that came out too dark or darken the hair that is too light.
To Lighten Dark Hair
Choose a color corrector product. Some require you to mix the product with the developer. Be sure to read about the product and make sure you have all the necessary supplies.
Mix the corrector, as directed, in a bowl. Use on dry hair (or wet if your product so directs you). Apply the mix with an applicator brush to your hair only where the hair is too dark. Your corrector product will give you a time range for leaving on the product. Decide how long is the right length of time; only you know how much "dark" you are trying to lift from your hair color. If you are unsure, leave in the product for the minimum length of time.
Wash out the color corrector, and dry your hair. If your hair is still too dark, repeat the application.
To Darken Light Hair
Select a semi-permanent hair color that is the same shade as the darker hair you wish to match. You could do an additional treatment with your permanent hair color, but that will be really harsh on your already damaged hair. Semi-permanent colors are more gentle. Read the product description. You want one that primarily deposits color -- without using any bleach or lift/developer. Pour (or mix if required) the semi-permanent dye in your bowl.
Apply the gentle product using an applicator brush to those sections of hair that are too light. Process as directed by the package. Again, your product will give you a time range to leave the semi-permanent dye in your hair. Only you know how much color you wish to add to darken your hair. The more color you want applied, the longer you leave on the product. As before, you can always opt to use the product for the minimum time and reapply if necessary.
Wash out the dye, and dry your hair. If your hair is still too light in places, you can repeat the process.
Correcting Damaged Hair
Select a cholesterol conditioner. These products are generally concentrated conditioners, designed for dry, damaged hair. Since uneven hair color usually results from damaged hair, repairing your hair can ensure that the next time you dye, the color will come out more evenly.
After you have washed your hair, towel dry it and then apply the cholesterol cream conditioner liberally over your hair, focusing on the damaged ends.
Put your hair in a plastic cap so the cholesterol can process with the natural heat provided by your head. Leave it in your hair for at least 20 minutes.
Rinse out the cholesterol conditioner, and dry your hair. Your hair should already feel stronger, softer and less dry. You can repeat this process every day or however often you feel it is necessary to help repair your damaged hair.
Use your cholesterol conditioner after you have made your color corrections. You can wait and do it the next day. Conditioning your ends will help ensure they do not fade unevenly. Or at least they will fade less unevenly, for a more natural look.