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. They produce a mucousy substance that slows dehydration. Mucus glands keep the skin moist and allow skin breathing. Granular glands secrete toxins - This is why dogs spit out toads. Reptiles use keratin to waterproof.