open the .html and add "|| color === tileColorStartIndex " to
var isTransparent = (color === 0 );
like so:
if (color != fillColor) {
var isTransparent = (color === 0 || color === tileColorStartIndex);
imageCtx.fillStyle = makeFillStyle(color, isTransparent);
imageCtx.fillRect(x * imageScale, y * imageScale, imageScale, imageScale);
}