Which type of corneal opacity can appear yellow or green and is a sign of infection?

The presence of yellow or green corneal opacity typically indicates an infection, and this is most commonly associated with white blood cell infiltration. In cases of corneal infection, such as keratitis or bacterial infections, there is an influx of inflammatory cells, particularly neutrophils, which contribute to the characteristic color change in the cornea. The appearance results from the accumulation of these cells along with the byproducts of inflammation, leading to turbidity and altered coloration in the cornea.

Corneal edema, while associated with fluid accumulation that can cause a cloudy appearance, does not usually manifest with a distinct yellow or green tint specific to infection. Corneal neovascularization involves the growth of new blood vessels into the cornea, which may indicate chronic inflammation or irritation, but it doesn't inherently produce the yellow or green opacity indicative of an active infectious process. Corneal melting refers to tissue degradation due to severe inflammation or infection, but this process leads to a different presentation, often described as corneal thinning or ulceration rather than a color change related to cell infiltration.

Thus, the association of yellow or green opacity with infection in the cornea is best attributed to white blood cell infiltration, making it the correct answer.