As far as I know, these cannot be changed. If there was any hacky way to change them, I'd think it would be in the XML of the TWB file, but I've not heard or seen anything about changing them. AND changing the XML in a TWB file is always unsupported by Tableau, so I'd say its probably not worth the effort if you can accomplish your overall goal in another manner.
What is your overall goal in changing these colors? How would that benefit your analysis? I'm just trying to understand the overall goal, as there may be another method for obtaining the results you seek.