Spiders of many species change color to match the pigmentation of the flowers they sit on  The insects preyed on by those spiders, unlike human beings, possess color discrimination so acute that they can readily see the spiders despite the seeming camouflage
Clearly, then, it must be in evading their own predators that the spiders' color changes are useful to them
The color discrimination of certain birds that feed on color-changing spiders is no more acute than that of human beings.