While playing around with poke/peek, I discovered theres a new palette to choose from.

Poking the address 0x5f11 and setting it to a value of 0xf1 [the 1 being the color in the palette, in this case dark blue] you'll notice something strange. The blue seems to be even darker! this goes from 0xf0 to 0xff and corresponds to the color palette available naturally. It seems to darken/add a warm filter to the colors, allowing us to provide sprites with more specific details. You can see this in action via Neil Popham's post on twitter. To return the color to normal, replace the f before the color ID with 0. for example, 0x01 would be normal dark blue where as 0xf1 is the darker dark blue.
https://twitter.com/ntpopham/status/1169166881607077888

Address range is 0x5f10 to 0x5f1f from what I can tell, and there is no other hidden colors yet discovered but knowing zep... there probably is. :)

Have fun with this discovery.

Seems like you can get a few extra colors by set but not by individual of the 16.

However since at least sets of 4 can be done HERE, it is likely ZEP will add some new commands in future PICO-8 for palette tweaks.