Wow, ages ago, one afternoon, I played around with "page swapping" on the Mac Plus. You could cordon off a chunk of RAM to act as a second screen buffer, render to it, and then toggle the screen buffer during screen retrace to get a very fast (60hz, er 30hz, I guess) "flip animation".

As I recall from my brief experiment, you could get a pseudo 50% gray where the corresponding pixels on the two buffers were opposites (one buffer with a black pixel, the other with white).