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?