Since I am targeting 60fps one of the things I can do is flash a black sprite every other frame to get a shade of grey. Yes! Grey on a black and white screen.
It only works at high frame rates (note: 50fps isn’t high enough!) because the screen is so damned good that flashing at a slower rate simply looks like… an image flashing. The effects is also visible on device for the same reason.
So I’m not sure I’ll use it, as all other shadows are dithered and flashing them all will hit the frame rate. But it was fun to try! One for another game, perhaps?
I believe this technique was used on other systems with limited colours, such as Game Boy and Palm (to get 4 shades), certain Commodore 64 games (eg. Creatures 2) did it to get never-before-seen colours, and of course old arcade and video games often did exactly my trick to get their semi-opaque shadows.