One thing you could maybe do is converting you image to a SVG.

You would then have a path for your car windows with some class. You could afterward change the style of the SVG class in JS.

To fix the problem you want background color white. As your link there are a function called removeTransparency it makes the image background color white see it.

      // make all pixels opaque 100% (except pixels that 100% transparent)
      function removeTransparency(canvas) {
        var ctx = canvas.getContext('2d');

        var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
        var nPixels = imageData.data.length;
        for (var i = 3; i < nPixels; i += 4) {
          if (imageData.data[i] > 0) {
            imageData.data[i] = 255;
          }
        }
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        ctx.putImageData(imageData, 0, 0);
        return canvas;
      }