These are our DIY invitations. Well, we had them printed, but we designed them ourselves and we love them. The thank-you card will be in the same mood.
Outside: light pink and grey on dark pinkish/wine red
Inside: dark pinkish/wine red on ivory
Inside on the left it says: "I'm happy that I found you, otherwise I would have searched for you all my life." (the translation is not perfect, but you get the idea).