Why does mixing yellow and blue paint usually make green?

Mixing yellow and blue paint creates green because paint mixing is a subtractive process, not an additive one. Unlike mixing beams of light, which adds spectral components, pigments work by absorbing specific wavelengths of light and reflecting others.12

A yellow pigment reflects yellow, green, and red light while absorbing blue and violet wavelengths. Conversely, a blue pigment primarily absorbs yellow, orange, and red light while reflecting blue and some green light. When these two pigments are combined, the yellow pigment absorbs the blue light, and the blue pigment absorbs the red and yellow light. Green is the only spectral component that is not strongly absorbed by either pigment, so it is the color reflected to the eye.12

It is a common misconception to view red, yellow, and blue as the fundamental primary colors. While these are often used in art, the more accurate subtractive primaries are cyan, magenta, and yellow, which provide a greater gamut of colors.13