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?

I've watched the first 6 episodes so far, and I've already noticed several scenes where his eyes are "normal" rather than blue...

I'm pretty sure the blue eyes are contacts (not CGI), so perhaps he had some problems with irritation and they had to film those scenes without them?  But if that's not the case, then it's a gross oversight by everyone involved (especially the actual actor).  If contacts are part of your daily "costume", that's not something you would just forget.

So, yeah, I'm not sure what's going on with that.  There might be a "reason" for it that's explained later on...  Perhaps the eyes fluctuate between blue and "normal" as they get close to their 20th birthday?

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.

My random thought was he is "damaged" already it could be an effect of his "powers" not being as strong as he is half human/ half wing man. OR he's eyes are blue when he is feeling stronger emotions- tbh tho I did not look into that thought just was an idea I had was watching.