A tiger seems like the least discreet creature in the jungle. That intense orange, those black stripes, everything screams presence.
But that is only true for your eyes. You are trichromatic, like almost all primates: you have three types of cones that distinguish red, green, and blue, an advantage that scientists believe evolved to detect ripe fruit among the leaves.
Deer and wild boars were not so lucky. They are dichromatic, lacking the receptor for red and orange, so their brains turn the tiger into an olive-green blur, almost identical to the foliage. The stripes do the rest: they break up its silhouette and blend it into the shadows.
Mammals cannot even produce green pigment, so nature used the color they did have available, and it worked just as well. It is the same reason hunters wear orange vests: to you, they shine from miles away; to a deer, they are just another bush.