Originally posted by The Unsung Hero
After a 3rd reading of it I think it works like this.
-Ash is a Fire-Aspect Dragon-Blooded
-Ash enters Water Aura
-Ash uses Currents Sweep to Sea and gains a bonus for being in Water Aura
-Ash wants to use Earth Bears Witness but cannot because he is in Water Aura.
-Earth Bears Witness has the Aura (Earth) keyword and cannot be used outside of Aura (Earth)
-Ash can purchase Earth Bears Witness a second time to get a variation with the Water Aura, OR can change to Earth Aura to be able to activate Earth Bears Witness.
I think every DB has access to every Aura. If Ash were in Fire Aura, the mechanics seem to imply that Ash would get a bonus using some Fire-Aspected charms because Ash is Fire-Aspected. We don't see any of those examples.
This is my take on it.
You've got the right of it.