If searching in the ROM 16-20-21 doesn't work, it's probably hardcoded.
Try looking around for A9 16 ** ** ** A9 20 ** ** **
(that is LDA #$16 STA ???? LDA #$20..., possibly STA $2007 to write directly to palette RAM, or else to somewhere in RAM if the game buffers VRAM write data to RAM before writing to VRAM)