The problem with taking a Rainbow Pretender as Ermor is that your Pretender will be the only mage you have who can make use of the gems that you find, since Ermor's mages are among the most limited that I'm aware of. So most of these other gems will probably just get converted to Death gems. That's useful, but at a 4x1 cost (2x1 for Astral gems) is it really worth building your Pretender around? Probably only over the very long run.
My advice is build a Pretender that helps you more over the short-term, and find your magic sites by taking over someone else's territory.
