Think about your hair, is your hair dark or light? All you have to remember about the Ombre look is contrast. So, if your hair is light, add in black, brown, red, pink, blue, etc., on the bottom. It could be gradual or not. If your hair is dark, you can add blond, red, blue, pink, etc., again it's all about the contrast. Some people like having your hair look out grown and some like the gradual color all the way down the hair shaft.
If you are going to try this, make sure you take the length of your hair, especially your layers, in consideration with your extensions. The extensions need to be able to show through as your bottom layer, this means that your layers have to be cut especially for your extensions. Section your hair up at the bottom, by taking two inches from the bottom and clip your hair away, clip in the extensions. If your hair is healthy you may need to lightly backcomb the root area so that the clip ins will not slide through the hair shaft. Take down a couple of inches more and do the same thing, teasing at the root before you clip in the extensions will help.
Watch this video to help you add in extensions.
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