Original: This color-changing function is produced by groups of red, yellow, brown, and black pigmented chromatophores above a layer of reflective iridophores and leucophores, with up to 200 of these specialized pigment cells per square millimeter.
Simplified: This color-changing function is produced by groups of red, yellow, brown, and black pigmented chromatophores above a layer of reflective iridophores and leucophores, all of these being parts of the cuttlefish's skin which help it change its color. There are up to 200 of these special pigment cells per square millimeter.