Cyanosis is blue or purple coloration of skin or mucous membranes, due to tissue near skin surface having low oxygen saturation. Signs usually first appear on the lips or fingers.

• It occurs when >5g/dL of deoxyhemoglobin is present. As such, if there is a low hemoglobin count, such as in anemia, the bluish color will be less apparent. It is also more difficult to detect on deeply pigmented skin

Source: Simple booklet

Source: Blogspot