While it is allowed, the sprites themselves have shading; you can see it on the diamond: https://v6p9d9t4.ssl.hwcdn.net/html/8532817/atlas.png
It can be fixed easily, though; just dither it with Aseprite or any other pixel art tool.
Ok, my bad.I thought it was OK since the "shading" is just applying one color over the over with a 50% (or so) opacity.I will know for the next 1 bit Jam :-D.
