The original Undertale relies heavily on 8-bit and 16-bit inspiration. While overworld sprites are colorful, the battle screens are strictly black and white.
The mod became popular on social media (Twitter/X and Reddit) because screenshots look official. You can finally see the "Abyss" in Waterfall as deep blue, or the DT Extraction Machine as a crackling magenta. undertale colored sprite mod 108 high quality