Wow! Beautiful piece! But just a little bit of advice for a critique: the abundance of shines is a bit much to take in. All of those white spots are distracting from your amazing coloring! If I were you, I'd think about narrowing down the spots to keep them all. For Luna I would narrow them down to the ones on the horn, on the cheek under her eye (curving downwards a bit, to follow the contour of the cheek), the one on her forehoof, and the one on her wing. The ones in her hair would look nice if elongated and instead of circles I'd make them into long highlights following the darker lines in her mane that you've made. I'd also do a longer highlight on the chest piece as well, look up a few pictures of curved metal and study how the light looks on it. As for Celestia, once again, I'd do long highlights as well.
Though your grasp of the MLP coloring style is great! And the subtle change of shades in the eyes really bring the characters to life. I love the texture in their manes and all of the different tints you've been able to include in what are normally flatly colored characters. They both have a quality about them that makes me just want to take them out of my computer screen and hug them!
*gasps* Thank you for the big and awesome comment! You made me smile and I feel so happy! Also, thanks for advices. I'll try to get better in the future. And I rarely get critique and really appreciate it! And again, thank you. Thank you so much! ;_;
Hah! No problem! I wish I got critique more often as well, I'm working towards being an illustrator for graphic novels and hardly ever get serious critique so I know how hard it can be. I'll add you to my watch in case there are other tips I can pass along. Keep at it and good luck! You're doing great!