Mucus cells produce mucus, poison and in some fish produce light. Proteinaceous cells make various substances like slime, enamel and other things and produce keratin which is used for waterproofing and in feather, hair, claws, scales and the outer stratum corneum. Chromatophores are color cells found in the skin. They can be epidermal or dermal. Birds and mammals have melanophores with melanin in the epidermis.
Color change is slow and caused by the production of more melanin - morphological color change .
These may be involved in morphological color change or physiological color change which is rapid and caused by shifting of the pigment or crystals in the chromatophore. Jawed fishes have thin skin. Most lack keratin. All fishes secret a slimy protein. Amphibians have a thin stratum corneum.