I don't expect them to have different colors, even if their roles in the meta-narrative differ.
I mean Deltarune determination is associated with the light world while Undertale determination is associated with Humanity. So that may be why.
I think her eye glowing red has something to do with the knife itself somehow... at the end of chapter 1, Kris' eye glows red, and they don't make a fountain. The only thing they're determined to do that night is eat a lot of pie.
Red could still be determination, but just thought I'd point that out.
Oh! Something I forgot, but how would this fit in with the funny thing of Toriel in Undertale having dark red eyes in her sprites? Would that be more evidence for determination being red due to Boss Monsters being specifically highlighted as having souls that briefly persist after death unlike other monster souls?
Maybye, but it be Golden and Red as The Scheme, Like the SOUL and Chara’s Text, representing hope and its absence and Flowey with the Lack Of Hope but Determination of seeing everything in the game or the static of save points. Seems like a good parallel and is a Color scheme that would work.