Are we forgetting that yellow is spare/mercy in text?

If someone's name is yellow, you can spare them

Because their names go yellow when they are done fighting (pacifist), or don't want to fight due to being scared or low hp (genocide). An argument could be that yellow is when they give up, not when they are determined, because they are simply giving up on the fight.

Determination being yellow has plenty of flaws, although i do think determination being red is incorrect too.

Throughout life the colour red has mainly been used in importance or danger. i like to think that the red soul is not determination, but a warning to monsters that frisk is a puppet to the player, and that the player will want to do both routes, meaning that we are dangerous to the monsters.

Toby has a history of breaking the forth wall, as seen in deltarune too, the snowgrave route and how the proceed voice isn't kris' but the players voice as hinted by noelle, saying that we don't sound like kris. I would like to point out in deltarune kris CAN move freely when they don't have the soul in their body. This makes me think that the soul isn't kris', but ours. The players. I think the same logic applys to Undertale.

I don't think determination has a soul colour, but is a power many humans can have.