Starting with an opponent colors formulation of color vision, two parameters, t and d, may be defined that express an illuminant's ability to realize red/green and blue/yellow contrasts of objects. For instance, calculation of t and d for daylight shows that on a grey day, color contrasts were actually reduced. By these measures, many common vapor-discharge illuminants systematically distort object colors. Because red/green contrasts contribute to border distinctness, and both types of color contrast contribute to brightness, such systematic distortions probably affect the overall clarity and brightness of what is perceived visually. Experimental data are consistent with this idea. In relation to color constancy (retinex) experiments, it is approximately true that the visual system discounts the color of an illuminant but not its t and d. -- AATA