The CIE 1976 L*a*b* Color Space, CIELAB, has been widely and successfully used in a variety of applications including digital color imaging, color image quality, and color management. One of its shortcomings, lack of hue linearity, is a critical problem in color gamut mapping and has been addressed by the IPT color space which is widely used in this domain. One limitation with both of these spaces is their applicability to color problems in high-dynamic-range (HDR) imaging. This is caused by their hard intercepts at zero luminance/lightness and by their uncertain applicability for colors brighter than diffuse white. To address these HDR questions, two newly formulated color spaces are proposed for further testing and refinement, hdr-CIELAB and hdr- IPT. They are simply based on replacing the power-function nonlinearities in CIELAB and IPT with a more physiologically plausible hyperbolic function, known as the Michaelis-Menten equation, optimized to most closely simulate the original color space for the diffuse reflecting color domain. The formulation of these proposed models is described along with some preliminary evaluations using Munsell data in comparison with CIELAB, IPT, and CIECAM02.