Hey, I made this pixel-art 3 weeks ago, so I hope you like it.
There is 12 colors, in this pixel-art:

What can I do for make this PA better?

a body would be nice :)
barring that, there are some things you could do to clean up the head.
You've got aa-banding going on on the right side of the hat (aa-banding is explained in the PJ tutorial, found in the resources/support forum, if you're unfamiliar with the term). one eye seems to be much closer to the nose than the other. the beard looks solid white, you need more contrast if you want that shading to be evident (meaning darken those greys a bit). there's more banding where the beard meets the face (the banding being the grey pixels). his side burns seem to drift away from the head and towards the ear a bit much. design-wise, a larger (thus goofier nose) could potentially give him more character, but that's just an idea. the general construction of the head seems a bit wonky, it looks perhaps like you drew all these parts together as they now are, rather than construct a basic head an put the beard and other features on top of this base? Good construction in the early stages goes a long way. his hair doesn't make a lot of sense to me, it looks like a fringe that follows the edge of his cap rather than a natural hairline. The volumes of the face could be better defined by using light and dark pixel clusters places according to a light source, rather than scant line work (and no highlights). the glasses are strangely-shaped, the 'arms' of the glasses curve up dramatically, rather than extend straight out from the frames. the bridge of the classes doesn't seem to rest on the nose as it should. his face is a bit short, there's no room for a mustache, the nose and mustache seem to be competing for the same space on the face, and his mouth should be lower.