static BufferedImage bwImageAsAlpha(BWImage bwImage, Color color) {
  int w = bwImage.w(), h = bwImage.h(), n = w*h;
  int rgb = colorToInt(color);
  byte[] in = bwImage.pixels;
  int[] out = new[n];
  for i to n:
    out[i] = ubyteToInt(in[i]) << 24 | rgb;
  ret bufferedImage(w, h, out);
}