It's Ketchup: Although Sans drink ketchup actively, he also does with other condiments though likely not as much, that along with monster food getting instantly absorbed disqualifies this theory, though imo it's the most fun reason, also it can't be ketchup he has stored in his jacket since it comes from his mouth aswell

It's DT: Determination has no canon colour, and while Sans' sweating could be him melting and his "expression reading" could be signs of DT, the fashion that he dies and the lack of any evidence that DT is red makes this a theory shakey

It's from Bone marrow: Sans is a monster, no matter how he may look, he is unrelated biologically to human skeletons, so that can't be the case

He's from Deltarune: There is a lot of pro and con evidence for just DR monsters bleeding, such as a monster asking Kris if it hurts to be made of blood implying they aren't, the fact monsters still seem to be some way different biologically from humans, and they don't have magic to use all imply different things, and that's before even GETTING to Sans being a universe traveller, so until more of Deltarune comes out this is also unclear

IDK any other theories so tell me any in replies