OK, here's what ya' do. I'm licensed, so trust me. Go get some peroxide 20-30 volume. Put some in the palm of your hand...a good dollop...and mix with some shampoo and wash your hair. Leave it in a few minutes and rinse. Do it a couple times and you'll see the color washing down the drain. It won't lift your natural color, just the deposited color. Then put some good conditioner on and leave a warm towel on it awhile to get the condition back. I like to microwave a damp towel to heat it. You can do it a couple days to lighten the deposited color and it will speed up the 28 shampoo process. It's called a soap-cap.
Good luck!