A tip on macOSXHints shows you how to create a coloured signature in Apple Mail.
This only works, obviously, if you email in Rich Text.
In the comments, another poster links to the older macOSXHints tip on creating more complicated HTML signatures , saving them as web archives and manually copying them into your Mail folder.
Or you could follow this tutorial on inserting a graphic into your signature provided by a Skype user, but useful for any graphic not just Skype promos. (You could, but now it requires you to log on with a Skype account. Shame.)
It goes without saying that restraint and good taste are crucial in this matter. Don’t do this.
Tags: Apple Mail, Apple Mail Tips, color, colour, email signatures, HTML, mail.app, signature generator