Run the cart and you will see more colors that are not in pico 8 how I got them is I drew a square of one color and then a square of another and used _update60() to change them really fast and it goes too fast for our eyes so it looks like the colors are combined.
the squares on top are the solid ones and the ones on the bottom are checkered. the ones in the middle are normal pico 8 colors.
feel free to use this in any of your games.

color pallet trick cool gradiant tutorial update60