Yves Klein made a completely blue stamp and used it as postage on an invite for a 1958 show.
A single perforated postage stamp painted with Klein’s famous IKB (International Klein Blue) pigment. The stamps were originally created to use on invitations for his simultaneous 1957 exhibitions in Paris at the Gallerie Iris Clert and Gallerie Colette Allendy. This unsent stamp retains its original blue luster. Mail Art scholar John Held, Jr. writes: “With the mailing of his blue stamp, Klein not only created one of the first artist postage stamps, but also involved it in a postal action, prefiguring not only the widespread genre of artistamps in the following decades, but the conceptual philatelic actions of the Fluxus group in the sixties…”
Skyline Books
A couple other examples:
artgallery.yale.edu/collections/objects/169825
christies.com/features/What-inpsired-Yves-Klein-9153-3.aspx
Originally discovered via @jacindarussellart’s Instagram
An unused stamp goes for $2,000 to $3,000