Amadeus is the official developer of the application, but it appears there is no information related to the way it works. I couldn't find any resource to get the necessary details on how to change the color of the emails. I recommend accessing the HELP menu within the software. Go to Help > Contents and switch to Search. Type color and then you should see the results related to this query. Additionally, use the contact details to get in touch with the developer to ask him about this request.