It is an enigma for students why artists use red, yellow, and blue as primary colors, whereas physicists use red, green, and blue. To answer this problem, the spectra of a series of mixtures of red, green, blue, and yellow tempera (poster paints) were obtained using a Carey spectrophotometer with an integrating sphere. Points on the chromaticity diagram were obtained from the spectra, showing that whereas the mixture of yellow and blue yielded a fairly saturated green, the path of mixtures of green and red traveled on the blue side of the white point. Hence yellow is preferable as a primary color than green for tempera.