I don’t know how it happened, but in my message list, one message appears in green font. The right-click menu for that message offers no way to un-flag (?) or un-color it. I don’t know what green means. I can find nothing in the help that’s relevant.
Does “Assign color” in the context menu help?
No. The color-me-white box (not the green box) has a checkmark in it. I re-selected white, but there’s no change, no effect.
Hmm, hmm. 
Under “Edit > Preferences > Mail Preferences > Labels”, is there a label with a green color? If yes, then try “Labels” from the right-click menu on that message. If I remember correctly, labels can have a single-key digit shortcut so they can be easily assigned accidentally.
Thank you. That was it!