I might have to disagree that it can't be done...it just may not be easy.
In the preferences you can define different colors for certain senders in the attention indicators section and it is "dynamic" so to speak and doesn't require the user to have the designer to do so.
Maybe the button can add a field to the document and the color column can be used in conjunction with the field? I don't have a notes client in front of me right now otherwise I would try to offer a bit more code