Coloration of snakes is largely due to pigment cells and their distribution. Some scales have lightly colored centers, which arise from regions with a reduced cuticle. A thinner cuticle indicates that some sensory organ is present. Scales in general are numerous and coat the epidermis, and they come in all shapes and colors. They are helpful in identification of snake species. Chromatophores in the dermis yield coloration when light shines through the corneal layer of the epidermis. There are many kinds of chromatophores. Melanophores yield brown pigmentation, and when paired with guanophores, yield grey. When paired with guanophores and lipophores, yellow results, and when guanophores and allophores are added to melanophores, red pigment results. Carotenoids also help produce orange and red colors. Dark snakes (dark brown or black in color) appear as such due to melanocytes that are active in the epidermis. When melanin is absent, albino individuals result. Snakes do not possess blue or green pigments. These arise from guanophores, which are also called iridocytes. These reside in the dermis.