It isn't, really. The color yellow is a purely human interpretation of a collection of radiation that hits the retinae of our eyes and gets processed by our visual cortex.

Animals seem to have some analogous way of discriminating light by some kind of 'coloration' sense, but we have no way to know if their brains actually 'see' the same thing we do when they look at 'yellow' light. Color detection is caused by photosensitive cells called 'cones', and some animals have none of these and would see the Sun as, presumably some shade of grey since there retinae have the black-white sensitive 'rods' instead, and which have a greater light sensitivity.

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