It's a technical issue (sort of)
Original Source - Quora
Due to technical mishaps involving Bloom's contact lenses, in the films Legolas' eye colour sometimes changes between brown, purple, and blue. (In the director's commentary of the Extended Edition, Peter Jackson admitted that they forgot to put Bloom's contacts in several times.)
Orlando hated the contacts he wore in the Rings trilogy because they stung his eyes after a long time of shooting the film. So in Desolation they just colored his eyes blue in post-production.
PJ changed Legolas' eyes in the Hobbit to distinguish him from from the Silvan elves of Mirkwood, as Thranduil and his son Legolas are both of the Sindarin. They are also known as "grey elves" and were kind of treated like nobles by the "lesser elves". I think the lighter eyes are especially light this time around just to emphasize the distinct ancestry.
The Hobbit movies were shot in 3D 48fps. In one of the production blogs it is explained that colors tend to saturate out a bit (hope I am using the right language here) when shooting this way, so they had to make everything slightly more colorful, to seem normal on screen