Board Thread:Wiki Discussion/@comment-44601210-20191206012456/@comment-35030974-20191219054144

I think red represents Purpose, and if you watch Underlab, you already know why.

Combining yellow (Justice) and magenta (could pass for purple, Perserverance), gets red.

Justice could well represent deciding who lives and who dies, which is what Frisk does on every route, even if that means killing or sparing every monster. And we already know what's Perserverant about Frisk.

Chara shares the same SOUL, and they are called 'the future of humans and monsters' multiple times. People with red SOULs serve a purpose, whether it be to kill every monster or set them all free.