Wow! I hope to make constructive criticism. I am honored to make one of this piece of art.
Well, in the first place, the thing that impresses me the most, beyond the vibrant colors, it’s the ability to color with grays and whites and create shapes. The points that seem to be more realistic are definitely the inner and outer corners of the eye. They express the true effect of “light” and “wet” of the eye seen up close. I really like how you've been able to spread the color of the iris, especially in the connection between the blue, yellow and green. Little brown spots complete all. The red instead seems a bit fake, but this only at the bottom where it appears much stronger than the upper, where it’s diluted and blurred. The sign of the eyelid is marked properly but perhaps a bit forced, unlike the eyelashes sprout in a perfect way from the eye. Another thing I find very well done are the veins of the eye. The skull should perhaps be a bit precise but I imagine that in the short space of the pupil wasn’t easy to make a complex shape.
I hope this critique will be useful (and I hope that I expressed well myself).
Good job, as always!