Thanks for the comments.

Yeah, it's very simple and flat, but that's how I normally draw faces (though in hi-res and with vectors). Glad you like it though.

RoboBOT: I think I prefer the white, but I'm still undecided. But I guess I'll make a non-challenge version of the face anyway..

 A nice original take on the theme. I like it, even though some aspects, like the lips, aren't so good.

M. Lucanius : OK, I wasn't sure :) My bad !

Nice, I like the simplicity of the face. I would change the white to the skin tone. You'd have to make the background skin tone as well, but you could make it fade to a lighter or darker color (by dithering) to make the skin of her shoulders still stand out. Here's what I mean

 Still, it's quite nice as is.

Because it's not a restriction... maybe? It says "you may" not "you must". 

Nice portrait, very flat though.

Why don't you respect this restriction : " You may use different colours for each quarter." ?

Thanks. Palette count is fixed.

That looks funny.  Nice work.

Black and white count as colors so the palette count would be 8