Please keep in mind that the TextBox actually is one control and you can either disable or enable it. You cannot allow users to control only a part of its value. Also, there is no application (as far as I personally know) that has such color editing component which exposes the functionality that you require. Users always are allowed to edit all the HEX representation of a color if they are allowed to edit the color at the first place.
Pavel R. Pavlov
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