Every species of penguin has a similar color pattern, with dark feathers on their backs and pale feathers on their bellies. This is a type of camouflage known as countershading and is found in many different animal species.
Since three dimensional objects would usually look lighter on the side facing the sun and darker on the side facing away from the sun, this kind of shading can fool the eyes of predators from a distance.
In penguins it also serves as a means of disguising them when they are swimming. Looking at a swimming penguin from below, its pale feathers would fade into the light from the surface. From above, its dark feathers would blend into the dark water depths. This helps them to avoid predators and fool prey species.
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