A synthesis of the author's recent work on color-order systems and color-difference evaluation is provided in context of current knowledge and practices. The development of a colorimetric model is demonstrated using Munsell "Celtic crosses" as a model of perceptual space. Issues surrounding color-matching functions, unique hues, the Helmholtz-Kohlrausch effect, and lightness and chroma crispening are addressed, as is the difficulty of reconciling a difference-based hue, chroma, lightness model with an Euclidean model. A new lightness scale and treatment of lightness crispening is proposed The results indicate that, despite problems, relatively simple modified opponent-color models provide good accuracy in predicting color-order system and supra-threshold small color-difference data.