Hi!
-One thing to keep in mind about Gameboy colours is the fact that one of the colors must be the transparency. To help you get an idea of this, just change your background from green to any of the 4 tints you have chosen.
-A lot of the detail in your piece is gone at 1x scale. The hair and face are close in color that the blend together.
-The readability of it is pretty low. I can tell what you are going for, but there seems to be a lack of intentional placement of pixels in the forms. (I.E. the arm on the right)
That is just my quick thoughts.