The objective of this article is to analyze different color matching functions (CMFs) obtained with three (650, 530, and 460 nm) and four primary colors (650, 565, 513, and 460 nm), using both monoptic and dichoptic central vision. This strategy helps to clarify (i) lack of additivity of brilliance; (ii) shift in maximum sensitivity peaks of CMFs when experimental conditions change; (iii) variations in luminance for the same reason; (iv) strong metamerism of the mixtures; and (v) differences of chromatic opponence between monoptic and dichoptic vision. The results obtained reflect two important facts: marked stability of the visual system, which allows the experimental conditions analyzed to be solved with an equal degree of success, and plasticity based especially on the balance of retinal illumination, which was maintained at an average of 40 trolands. The results obtained bring to mind an assertion made by MacAdam to the effect that the law of additivity when applied to luminance is not applied to measurements of brightness. Perceptively, brightness is not additive, and so CMFs should not be considered as significant functions in computing tristimulus values R, G, and B. (c) 2005 Wiley Periodicals, Inc.