Cyanosis does not occur in severe anemia because

Because reduced Hb is purple, a low aerial O2 saturation causes cyanosis. However, this is not a reliable sign of mild desaturation because its recognition depends on so many variables, such as lighting conditions and skin pigmentation. Because it is the amount of reduced Hb that is impoant, cyanosis is often marked when polycythemia is present but is difficult to detect in anemic patients.