Office 365 Change Font Color

You cannot use formatting in the Subject field. You can however change how Outlook displays messages based on say the Subject text: https://support.office.com/en-us/article/Automatically-change-incoming-message-colors-and-fonts-based-on-sender-subject-or-recipients-4efbf993-fb00-4f2c-9a3f-78e64e4455ec?ui=en-US&rs=en-US&ad=US

Or if you want to sent an email message that's emphasized in recipient's Inbox, you can use the trick detailed in the last response here: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_other/change-subject-line-font-color-for-outgoing-e-mail/679a7c6c-5b7b-4d19-81fb-ef5e911824d7

I am trying to create a mail flow rule that would added a line to the top of the message body in red text but only found and option to modify the subject line

Again, you cannot use formatting in the subject. If you want to insert (colored) text on top of the message body, use the Append a disclaimer action for the Transport rule as detailed here: https://technet.microsoft.com/en-us/library/dn600437(v=exchg.160).aspx

Thanks but the problem with the disclaimer is that is goes at the bottom of the email. I need it at the top...