This is how my eyes work in real life. Pictures and cameras commonly show up blue but in person, I am a mix, certainly never as blue as that. Granted, I am not an emerald green but they can get rather green in warm light. It's because my iris is brown around my pupil and blue around the outer edge.
So he could have this going on. But the prevalence of purple eyes is extremely low and we've now seen three of them. Elizabeth Taylor's children couldn't get her eyes (which commonly show up blue in pictures and on camera.) And then, Paula's sister's coloring is near unheard of too. So, grain of salt.

Ok, I'm confused. The novel mentioned that Vincent's eyes are "His emerald-green eyes, strikingly vivid, seemed to pierce through to her. As their gazes held, Paula realized with stark clarity: His eyes could truly see"...
meaning, Lucas eyes are green right? So no more blue-eyed Vincent?