Column level access to allow users to update only certain columns on a custom entity

I've got a need to permit a group of users access only to certain columns on a custom entity.

Is there a way to do this using column security? 

Would users who have read/write access to the entity via a security role retain that access, or would I have to explicitly assign them a custom profile to preserve their access to the columns with column-level access?

Thanks,

Jessica Weissman