Why do mixed paint colours often become less saturated, and why doesn't blue and yellow always give the same green?

Mixing two pigments subtractively removes more of the light spectrum than either pigment alone, which is why mixed paint colours usually end up less saturated ('muddier') than either starting colour. The exact result also depends on the specific pigments used — two different blues and two different yellows do not have identical reflectance, so 'blue plus yellow' does not give one single, fixed green.