A gamut size metric is used in all phases of color image processing (e.g., capture, transmission, and display). In general, the gamut size metric is a single-valued metric that changes as image content changes. More particularly, a gamut boundary histogram is determined and used to estimate a gamut size metric. A gamut size metric identifies a minimum size gamut needed to encompass each pixel in an image, where the gamut size is limited at one end by a first device independent gamut (S1), and at another end by a second device independent color space (S2), where S1 is wholly enclosed within S2. The gamut size metric may be based on strict pixel color value differences. In other embodiments the gamut size metric may take into effect perceptual color differences and significance.