They can but they do not have to. Besides, rules change for DW anyway. I did boarding pass save the dates and message in a bottle invitations. I guess the theme is destination wedding? But our colours were there in the save the dates anyway, so it depends on you and what you like and what you want. Generally they should at least go with the FEEL of your wedding... sophisticated, relaxed, chic, fun, whatever it is that you are going for.