Wow, I just fell out of my chair. Anyway I'll work from the top down.

Hat:
-Define you light source dont out line it with a darker shade if its sosposed to be lit
-Propeller seemed to high up
-the shapes on the hat needed to be more triangle like
Hair:
-The front of the hair is very unnatural, It looks as though it just flops down.
Face:
-Theres no mouth! He cant breathe. lol
-More shading and also needs highlights
Shirt:
-The arm creates shadow over the body, darken it up.
-Put highlight on bolth the arm and shirt/sleeve.
Shorts:
-Very square
-Need highligts
Feet:
-Look rather big and round

As for color I would just use what you got there its always fun and enjoyable to see what you can do.

Here:


I outlined it in black so it can stand out againts the back ground.


Oh and good luck with the GM competition.

I move the mouth down and also used the lighter brown in the shadow of his face as a highlight for his hair.I think you need to use highlights instead of just useing darker shades. Also I made the mouth bigger (top and bottom lips). Added a white high light for his face.
As for the standing position I like it,looks good.



I fiddled around with the eyes and couldn't come up with something that looked good. I guess the soild black squares could work. They don't look half bad.

I'd sugest that you work on anatnomy.I'm not that good at it my self but I tried to edit it. Also you should think about chanding the arms into better figures and forming hands. Oh don't mind the colors I was just messin'.

He looks as if hes sitting in a plant pot. Add some cross paterns to convey a cone