yes, unfortunately the only options for the coral above ground is the cloud/water palette because it needs the blue for the water. and even if it didn't use the blue for the water, there's no way to get the pink/purple because there's no more room in the palette
Hmm, can alternative colors be included by inserting 'sprites' into the backgroud layer? So instead of it actually being a tile it being a static sprite, made 'inactive' as in you can't interact with it as an enemy. I think overlaying sprites were used in other games to create 'more color' at times, especially characters that needed to go beyond the limits of sprite colors.
It wouldn't be possible everywhere, as there is a limit to only three sprites of three different colors on screen at a time IIRC.
"three sets of three tile colors and three sets of three sprite colors can be used onscreen at once."
Though special chips might have bypassed some of these limits in some of the later games mappers, or finding exploits to do things system wasn't intended to do, but allow for extra stuff.
Of course there is always possiblity of using emulator only 'overlays' with emulators like Meson... Unfortunately that wouldn't ever work on hardware.https://www.zeldadungeon.net/check-out-this-fan-made-remaster-of-the-original-legend-of-zelda/
I wonder if once this hack code is finished if any of it is relevant and could be applied into design Super Mario Special for Super Mario All-Stars on the SNES? That could be a very beautiful game!
Or how about SMB Classic: Special, for the GBC (which I think added for more pallete colors, and extra onscreen animations, lava animations)? Not sure that version would be able to be backported to NES?