It's basically just not possible to prevent this. There are tradeoffs and problems with all sorts of problems. Censoring is generally something that requires some level of manual management. If someone is circumventing it, you should take action against them.
Would it be possible for a future version of Xenforo to add in editor permissions in posts in the same way as they've been added for signatures? This would at least allow admins to remove text coloring that bypasses the word filter.