Reduced it by 4 colors. Sorry, the sky gradient stays.

 Nice idea for a pic. XD

As "shampoop" was saying I also think you should loose the extra colors.

And you should look at some of the super good pixel portraits to get a idea of how they

define depth using contrast because it looks a bit flat also using ref pics can help TONS!

Here is a example of some basic edits using better contrast and a ref pic for the nose

--> i332.photobucket.com/albums/m349/fucbillgates/editorffff.png

Here is the same image with 16 colors.  I noticed you have 5 skin tone colors that are almost identical.

s3-us-west-1.amazonaws.com/adamjones/6eyesouless_abomination.gif