Bangle.js 2 Screen resolution in Flappy Bird

Is there a way to convert e.g.

var FLOORIMG = require("heatshrink").decompress(atob("iEKxH+kklABuLAAlgAAwNFB34OLmAAO0YAO5wAOA"));

to 3bpp without having the original png file?

Also if I change

  b = Graphics.createArrayBuffer(178,178,8);

to

  b = Graphics.createArrayBuffer(178,178,3);

I get

Uncaught Error: Invalid BPP

It looks fine in B/W though:

  b = Graphics.createArrayBuffer(178,178,2,{msb:true});
  var gimg = {
      width:178,
      height:178,
      bpp:2,
      buffer:b.buffer
    };

Edit:
I now understood that setting

  var H = b.getHeight()-24;
  b.setColor("#ff0000");
  b.fillRect(0,0,b.getWidth(),H-1);

instead of

  var H = b.getHeight()-24;
  b.setColor("#71c6cf");
  b.fillRect(0,0,b.getWidth(),H-1);

will stop dirthering, but how can I show some shade of grey? Is that not possible?

PS: I intended to let it run in full resolution - but that will be too slow. So I stopped developing that. I's be curious about the Grey thought :)