How to remove highlighting from emails in Mail?

I created a rule that applies a mint green color to a lot of emails in my mailbox. I no longer want that color on those emails and I do not see a way to remove this highlighting for even a single email. I know I could create a new rule to give them a different color, but there is no rule semantics for removing color either.

It seems that all of my email's subjects in Mac Mailbox are now permanently stained.