Here is something I've been playing around for a couple of days:
This cart allow to use a "higher" colour count palette for PICO-8, and of course, there is a trick here: You only have a canvas of 64x64 area as a group of 2x2 pixels are used for each pixel you can display using dithering.
This is mostly a proof of concept, I know I'm not the only one trying this. The code can probably be optimised, and the palette themselves are probably not perfect.
The bonus is that I have 3 variante of the palette:
The first one with 10 colours per channel eating the full first tile bank:
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A 8 colour per channel version:
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And a 6 colour per channel version:
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The difference between each is how much tiles are used for the palette.
I think the code is self explanatory, feel free to use any of these palettes if you see fit and to credit me if you use it :)
PS: cpc stand for "Colour per channel"