I was wondering why there are scenes showing Susie and Sans bleeding when monsters usually don't, and it occurred to me that blood might be the opposite of determination in Undertale and Deltarune. It's a natural substance for humans to have, but something that monsters only create under unusual circumstances. While determination manifests in moments of extreme hope and drive (e.g. Undyne the Undying) blood can appear in monsters in moments of complete despair (e.g. Susie seeing the end of the prophecy, Sans realizing that he failed and the world is going to be destroyed). Yellow text is used at times in Undertale, most often to refer to the actual SAVE power, but also in reference to having hope for positive outcomes in general (e.g. the names of monsters turn yellow when they can be spared). On the other hand, red text signifies a lack of hope, an indication that things won't end well (at least on the path you're currently taking) which ties into its frequent use on the genocide route.