Thanks. It would be nice if someone can make a GB utility tool that lets you add more than three colors to a tile.
That's... a hardware restriction inherent to the GBC.
All graphical assets in the ROM are developed around it.
Each palette can only hold three colors plus transparency.
Some developers did hardware tricks to show static images taking advantage of GBC's available thousands of 16-bit colors but it's rarely used (Crystalis GBC, some Telefang screens, and this game's title screen for the cancelled GBC remake) and definitely unworkable here for in-game segments because of the processing load.
There's dithering (can be implemented without any further hacking) but that wouldn't suit the art style of the game that well.
That leaves gradients (kind of like in the GBC Goemon game) which may be possible here.
Some emulators like GBE+ support sprite replacement with a bitmap with as many colors as you like but it's not romhacking the actual ROM anymore.