In fiction, when a character has yellow eyes, commonly the sclera, but can be the iris, it usually denotes that a character has a rather unscrupulous nature. Sure signs that a character cannot be trusted or is a bit of a Jerkass.
Historically, this is probably inspired by canines, felines, rodents, reptiles, toads, and birds of prey, which can have amber, yellow, gold, or orange irises, and are associated with being conniving, clever, and wise. Also reptiles and toads are sometimes associated with villainy, and can have yellow eyes. Ironically in these animals what you're usually seeing are larger irises which were probably mistaken for a yellow sclera. In particular, the visual itself might be related to the reflective eyes of many animals in the dark, which explains the particular bright palette in contrast with dark or dim figures.
A variant of this trope is a sneaky or crafty character with yellow or light-yellow eyes that have green or yellow green centers to them. Sometimes, the green-centered yellow eyes are used to accentuate the character’s sneakyness, craftiness, meanness, and/or evil in relation to non-green-centered yellow eyes or to distinguish them as the crafty or evil ones within works made up of largely or mostly yellow eyed characters. Non-green-centered yellow, gold, or amber eyes are more apt to be portrayed as normal or least less decidedly crafty or evil and are more apt to show up on good and ostensibly good characters.
In Real Life, yellow and brown in the sclerae or the cornea, not the iris, in human beings is unhealthy. It's usually a symptom of allergies or liver disease — yellow sclerae are a typical symptom of jaundice.
Compare Supernatural Gold Eyes, Black Eyes of Crazy and Red Eyes, Take Warning (the former two are commonly paired with this trope).
