Right but if yellow also stands for darker intentions and Peter's eye color is naturally blue then they combined to make the green eye since blue and yellow make green. It still holds to his good nature, even tho you can blackmail people in UD, but it shows more of a darker side has appeared because in Uxl which UD is based on Pito (which was Peter's original name) had two same colored eyes but once he was corrupted one eye changed.
Yellow is the eye color commonly found in predatory animals.
That is a lion's eye, by the way.
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And here is a tiger with yellow eyes.
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Dog with yellow eyes
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Wolf or Coyote with yellow eyes
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Fox with yellow eyes
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Owl with yellow eyes
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Jaguar or Leopard with yellow eyes
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House cats, probably the most observed case of yellow eyes
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But look at how beastly yellow eyes on a predator can be.
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Hawk with yellow eyes
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A husky with a blue and a yellow eye.
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Eagle with yellow eyes
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Snake with yellow eyes
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Bigger Snake with yellow eyes
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Shark with yellow eyes
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Crocodile with yellow eyes
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Cobra with yellow eyes
Therefore red can represent unnatural evil, while yellow represents the beast in his pants. Therefore, they do work together.
Also, think about this. Humans are most receptive towards the colors red and yellow. As humans are also more receptive to lowly pitched sounds than they are to higher pitched sounds. Why? Predators. Predators tend to have red or yellow eyes (bears can have reddish eyes) and tend to produce a lowly pitched sound like a roar or a wolf's snarl/growl.