static BufferedImage combineBWImagesToColor(BWImage img1, Color color1, BWImage img2, Color color2) {
  int w = img1.getWidth(), h = img1.getHeight();
  int[] pixels = new[w*h];
  int i = 0;
  for y to h:
    for x to w: {
      pixels[i++] = colorToIntOpaque(addColors(
        blendColor(Color.black, color1, img1.getPixel(x, y)),
        blendColor(Color.black, color2, img2.getPixel(x, y))));
    }
  ret intArrayToBufferedImage(pixels, w, h);
}