Why are his eyes are silvery now?

Besides Golden Alpha/Omega, there's another special trait/species which is Ghost and Diamond Dust. These two are believed as a myth as a very rare existence and they're made for each other.

Weikun is also a Ghost Alpha. Ghost Alpha can transform Beta to Omega, BUT in the condition the Beta is a Diamond Dust (which is Yeehyeon)

Not just knotting, Weikun had also transform Yeehyeon (Beta) to an Omega bit by bit. His blue eyes changed to white as a result of transforming Yeehyeon

This got so much hotter now lmao