Images can be broadly categorized into two types of image states: unrendered and rendered. Images in an unrendered image state are directly related to the colorimetry of a real or hypothetical original scene. Images in a rendered image state are representations of the colorimetry of an output image, such as a print or a CRT display. Over the years, many device-dependent, and device-independent color encodings have been used to represent images in both of these image states. This has resulted in a host of interoperability problems between various systems. There would be a significant advantage to standardizing on a small number of color encodings for the purposes of storage, interchange, and manipulation of digital images. While the recently agreed upon sRGB color encoding is one important piece of this puzzle, there is also a need to define standard color encodings that are not limited by the color gamut of any specific device. This paper will describe a family of new color encodings that have been developed to address this need. A new color encoding specification known as Reference Output Medium Metric RGB (ROMM RGB) is defined for representing rendered images, and a companion color encoding specification, known as Reference Input Medium Metric RGB (RIMM RGB), is defined for representing unrendered scene image data. These color encodings are both based on the same additive color space, which is defined by a set of imaginary primaries.