Okay, since I know nothing about computer coloring I didn't want to write you a critique at first, but now I think about it, why shouldn't I? I'm going to write this critique in steps like you did with mine, so it will easier to read.

First I want to talk about the characters. I think you made them look stand out, which is good, since they need to draw attention from the viewer. The way you colored them, is really simular to the way you color with colored pencils. I really like that, because that way one can see it's by you. I really like the use of shadows on Ash and Gary. It looks really amazing. For Pikachu and Kasuchu, you could have used a lighter color, to get a better contrast. Like how you blend colors traditionally. The colors always fit.

The background. Some aspects of the background look really great, but some are really bothering me. The sky for example looks amazing! I can see you put in a lot of time to make it looks this way and it turned out great. I love the "falling star" and the clouds in the lighter part look great aswell. Another thing I like is the grass Ash sits in. It looks so realistic! There is only one thing I dislike though. The trees. For example the colorscheme you used. You wouldn't find it in nature. I think you should look at the trees and bushes you make on your traditional work. They look so much better! I know for sure that it will work the next time you try

Overall, I think this picture looks amazing. I can see you tried your best and when you practice, you'll master it soon enough. I will always love your traditional work, but I love this too!

I hope I didn't sound harsh or anything, I didn't mean to. I only want to help you to get better at this ^^

Well, what can I say. I see it's really hard for you to color by PC, but I strongly reccomend you to use the Photoshop, Corel Painter (you can give art more beautiful traditional-looking) or SAI ( go to [link] - that program is free and special for anime fanarts)
I remember that time when I started PC coloring it was REALLY hard espesially picking the right color. I see that you go through your colored pencils habit and pick too heavy colors. I reccomend you to look on pokemon screenshots and play with them and dropper tool. and for night or evening you may create one more layer set opacity to 15 or 20 percent and fill it with dark blue or even violet that make picture look more realistic and soft. Of course you must erase the light in the character eyes to make them even more lighter. And I always use many layers with very high opacity to make colors and shades deeper as you do with your pencils. And really sad that I can see very small part of Mistychu and Gary is standing close to the center of image which draws my attention to his person and I can't see Ash because Gary takes all of it.
I'm REALLY love the anatomy especially Ash's hand and expression. Maybe it's just me but Pika-chan look a bit anthropomorfic on this pic maybe cause of his right paw (from his side)
Gary's expression is priceless xDDD I like it and Ash looks soo handsome Mistychu is beatiful but I can see just her eyes hairs and cheeks(
This is great art itself but I think you may re-color it now with layers (you still have clean lines, right?) I can give you superawesome coloring tutorial I found about coloring. Dunno, maybe it can be helpful [link]
I'm sure this art will be AWESOME with layer coloring!!! <3