Hi Customer, and welcome back to JustAnswer!
Your best course of action would be to have a professional stylist/colorist (or more than one) evaluate the condition and color of your hair and help you choose the best path to follow, to get back to your natural color. It's important to preserve the good conditon of your hair and strengthen it before taking any action. As you may already know, adding a 'process' and/or another color to your hair, if you have just dyed it the darker color, could cause damage, and you'd want to avoid that. Also, attempting to use a lighter color over the darker one will only darken it, so don't attempt that. Most colorists would attempt to lighten your hair with a 'color eliminator' and/or highlights, and the process may take a few months, until you're completely back to your original (or desired) color. If you feel this may be too costly for you, inquire into 'beauty schools' in your area, where students (supervised by licensed professionals) will help you get your hair back to the right color for a reasonable price.
If you would like to attempt this at home, L'Oreal and some other manufacturers make a
'color remover', which will leave you with an undesirable color that your can dye over, to the mid/light brown you want. However, keep in mind, this can be harsh to your hair's good health.
You can see more tips and techniques to help your situation here:http://www.wikihow.com/Dye-Your-Hair-Brown-After-It-Has-Been-Dyed-Black
I wish you much good luck!
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