The lineart is clean, but your colouring is, lacking some parts. Like an 'Umph' However compared to some of your other work I feel like this was rather quick compared to the others (Optimus).
But overall, I think your colours are somewhat flat I mean, compare it to some professionals like Chamba or some comic covers, I think you'll see that the highlights and refracted lights are different.
I'm no teacher obviously and not a professional, I'm just kind of eyeing some areas I see from UDON/Marvel Illustrators and then comparing yours.
But hey, this is in my harddrive now, which means I still really liked this piece!