While a number of color encodings are specified for imaging applications, neither the group developing the ISO 12640-3 CIELAB standard color image data, nor the ICC, found a suitable existing specification for a large gamut reflection print reference medium. Such a specification was needed as a color rendering target for the creation of the CIELAB standard images. Thus, the ISO working group started to develop the reference color gamut specified in the Draft International Standard ISO 12640-3. The ICC, in order to improve the interoperability issue encountered with ICC v2 profiles when using the perceptual rendering intents, adopted this reference color gamut for the perceptual reference medium for ICC v4 profiles. This paper describes the developmental requirements; the details of the specification of the reference color gamut as well as how to use it in the context of the ISO as well as in the context of ICC. It provides a more in-depth discussion than the ISO draft or the amendment to the ICC specification.