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In the world of hair, nothing is permanent -- not even that intense frost job that makes you look a tad bit whiter than you would like. Since coloring over frost requires the deposit of pigments into the hair, it is less damaging to your hair than removing pigments -- going lighter. So, you can dye over frosted hair, but there are some things you should consider.
Even though going darker is safer than other types of hair color, you can expect some damage from the process. For most people, the damage is minimal. If your hair is already on the brink of destruction, though, one more process might push it past the point of no return. You should never color over frosted hair if it is already falling out, feels mushy to the touch or refuses to dry. Doing so could make your hair fall out.
To prevent damage and ensure that the hair will hold color, you must "fill" frosted hair with a color filler before the service. The easiest way to do it is to purchase spray-on color filler from a beauty-supply store. You can also apply a bottle of liquid hair color without the developer mixed in it to the hair before the color service to fill it. If you choose the latter method, simply color over the hair with color mixed with color developer when you get done putting on the filler.
After a frost job, color fades from hair more readily. To prevent color fading after you go darker, use color-enhancing shampoos and conditioners that match the color you used. For example, use red shampoo and conditioner if you put auburn or burgundy on your hair.
Hair needs TLC after multiple chemical services. Condition your hair with a deep-conditioning treatment once a week, get regular trims and take extra care when styling your hair. Turn down the heat on your curling iron, flatiron and blow-dryer, and don't place too much tension on the hair when you style it.