Why does Apple Mail not provide default color font choice?
Apple Mail, since at least 10.4, has never provided a way to set a default font or color that will be seen by recipients.  The preferences settings are for local viewing only. Many have asked for this to be changed, but they clearly do not care. You have to use another email client or one of the addons which have been created to compensate for this (whose names escape me at the moment).