Your signature size is up to Outlook to interpret a baseline for small, medium, or large. Outlook will set the message font size and font to what the Outlook user has specified (default is Calibri 11 point in Outlook 2010) in the message body area. Open the message in Outlook 2013 for which you wish to change the font color.
If the font color settings are grayed out, then you will need to change the message format to HTML or Rich Text from the Format Text tab in the message window.
