Color mixing
Average mixing can be confused with subtractive mixing. For example, even though red and blue average to purple, an attempt to make a particular shade of purple by mixing red and blue paint may fail, since there is no purple spectrum between the red and blue spectra. Paint mixing can result in average wavelength. For example, mixing blue and yellow usually results in some shade of green despite them being complementary colors, because there is a green spectrum between the yellow spectrum and the blue spectrum. It's also one reason why mixing red and blue along with white can result in gray, because red and blue average to purple, but the average wavelength is green which is a complementary color of purple.