The point of using an enterprise CSM is to apply the company branding to
the web content that authors edit. Allowing authors to select custom
colors, like selecting custom fonts, sizes, background colors, etc, kind
of defeats that purpose. Instead, custom font styles can be configured
in the RTE that authors can choose from. With different CSS applied to
the font styles, it is possible for authors to set different colors from
a limited list. We'll however keep in mind for future RTE enhancements...