There were 2 rectangle sprites I noticed. The brown one is Dig Dug's shovel and the red one is his pump. If you look closely at the Dig Dug sprite when he is digging or pumping, you'll notice a little cutout in it. On a regular NES, the rectangles would be drawn behind the Dig Dug sprite and the color would only be seen through the cutout, if that makes sense. They use that technique in other games as well, though I'm not sure why.