I'd like to know why Tian Yi sometimes has normal eyes and then again blue. Is it just a sloppy job of the production? Or does it have a meaning?

A bit late but still had to write something. I think its more plausible that maybe he had some sort of problem wearing the lenses initially. In the beginning of the series there are parts in which his eyes aren't blue while everyone from the Wingkind has blue eyes. After that and until the end of the C-Drama his eyes are always blue. So, most likely wasn't being forgetful of it but maybe that his eyes took a while to adapt to wearing lenses.

Feng Ren (the regent king, the Emperor's uncle) on the other side never had blue eyes from beginning to end of the series. He was the only Wingkind whose eyes were always brown.