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many animals have counter-shading camouflage that reduces the contrast between shadowing and highlighting by using darker colors in areas that face towards the sky, and lighter colors in areas that face towards the ground - which in affect, makes them harder to see. this is also why studies have shown that users aren't able to navigate flat interfaces as quickly as interfaces with 3D elements. the windows 95/motif style is very user friendly, because the widgets are lighter on top, and darker underneath. the style is also abstract enough, that it never goes out of fashion. it is the universal interface style.