You could have the top part of the screen use a different color emphasis than the bottom, you would essentially have 50 different colors on screen at once.
I don't count colour emphasis for a reason : It is totally impractical to use it that way and would monopolize 100% of the CPU time. I only count colours which can be produced by the PPU without any CPU intervention.
Shame the NES never had a demoscene as that sounds like just the sort of thing they would do.
It has actually, check "High Hopes" or "YearsBehind" demoes. Blargg also did impressive demoes in regard to palette, and some demoes display all the colours with all the possible colour emphasises at once.
It has not reached the level of popualrity with C64 demoscene, most likely because it is impossible to stabilize the CPU with the PPU without obtaining a random jitter of at least half-a-dozen of pixels. On the C64 it is possible to get perfect synchronization (even if it is very tricky). Also the NES is really a gaming console, many tech demoes focuses on doing something that would be applicable to a game, while C64 is originally a computer.
Does emulators even support that?
Assuming your "that" is refering to colour emphasis, yes, most emulators does, except the very older ones like Nesticle. Some like FCEU supports (or used to support, it could have been fixed) colour emphasis but only on a whole screen at once, not changes during the frame.