You can create a new Mail rule to filter specific emails from a source and then apply a text or background color (but not both) to the message. Here is a new rule that I created to help order confirmations from Peets Coffee to stand out.


When you click OK, it will ask if you want to apply this rule to all occurrences of that From messages. In my case, my messages from Peets have that green background with white text. Ideally, I would have preferred black text but one cannot have both.
I didn't show a screen shot of the actual message as it had personal identifying information in it.