Can you simulate pseudo transparency drawing and not drawing the sprite between frames? GB Zeldas uses this techniques to simulate shadows and the Marin true ending.

Sorry, originally I thought it was okay, but I've been informed this is actually against the rules. Anything that gives the illusion of a fifth color is not allowed. While people did achieve this on the gameboy, it's not something you can simply replicate on modern hardware with accuracy, so it's best to just avoid it.