Blue or White Face
Blue is a recessive gene that removes all yellow, red, pink, and orange pigments from the feathers and leaves the blue, black, brown, and gray pigments.
In some species the blue gene is referred to by other names. In cockatiels it is called "White face" because the removal of the yellow and orange produces exactly that; a white face. In Green Cheeked Conures the blue mutation is usually called "turquoise." This is unfortunate as it leads to confusion with "parblue" mutations in other species which are commonly called turquoise as well, but are different from the true blue mutation that we see in Green Cheeks.