After reading Order of the Phoenix, I somehow got it in my head that Lily Potter worked as an Unspeakable studying love in the Department of Mysteries. I have no idea where I got the idea, and haven’t found any passage or lore indicating that this is canon, but I’ve been utterly convinced for more than a decade that this is fact, and have no clue of how to handle this information.
How do we know that Chara's soul was red? Furthermore, how do we know the seven soul colours we see in-game are the only ones?
In the basement of Asgore’s castle, there are six coffins with different colored hearts. These appear to be identifiers for the fallen children. On the coffin with the red heart, the name “Chara” is engraved. This is how we know that Chara had a red soul.
Nothing in game states that there are only seven soul colors. However, these are the only ones seen in the game. There is also importance on the number seven.
Seven magicians cast the spell.
Seven souls are required to destroy the barrier.
Seven souls make a monster god-like.
This is not enough to say there are no other soul traits, but seven is a significant number.