The usual (geometric) procedure to determine the chromaticity coordinates of a color that results from the additive mixture of colored stimuli is to plot the coordinates of these stimuli on the CIE chromaticity diagram and apply the center-of-gravity rule. This procedure, however, may become inaccurate and cumbersome if more than just a few colored stimuli are mixed in various proportions. An alternate method is presented that uses matrix techniques to solve problems related to colorimetry simply and accurately.