Why does Neymar have green eyes?

I have no idea who you are talking about, but genes are complicated. Skin tone does not determine eye color. The two are highly correlated, but correlation is not causation. So while some combinations of skin tone and eye color may be quite rare, they are all possible.

After a quick bit of research, it looks like you might be referring to the Brazilian football player. A lot of Brazilians are mixed race, whether they identify that way or not, and racial mixing is the most common way to end up with unusual combinations.

The vast majority of population in South America are “mestizos” which means that we are of mix origin (europeans, africans, and natives), so it is basically a lottery on how these genes present itself. Here is a pictorial representation of how such mixes were classified.