I think it will be difficult to find a completely automated way of doing what you describe (mostly due to potential complexities of ACLs). If your ACLs are not complex, you can do something manual with body classes and user styles.
I know of two plugins, none of which fixes your exact problem but both of which could either lead you to a different solution or help you build your own solution:
1. The
aclinfo plugin displays who has which kind of permission for a single page. That way your colleagues have a textual and not a visual clue.
2. The
loadskin plugin let's the admin define a different template per namespace (or page), not per ACL, though. If your wiki is structured in a way which gives clients access to only certain namespaces, you could use that instead. If you don't want to change the whole template, you can also copy the "dokuwiki" template (to, say, "dokuwiki2") and change some colours in there.