Determination is yellow
Determination is “the will to keep living… the resolve to change fate.” It’s the power that gives human souls “the strength to persist after death.” It is so powerful that it brought Flowey to life and gave him the power to control the underground’s timeline.
Because many people believe the red soul represents determination, some players associate determination with the color red. Many also incorrectly cite that determination is written in red in the genocide route. This is false.
Determination isn’t unique to the red soul. It is found in all human souls, and there is evidence that all of the fallen humans had the power to save, load, and reset. Furthermore, there is a strong link between determination and yellow – the save point.
In Entry Number 5, the word “Determination” is in yellow (and contrary to popular belief, “Determination” is never written in red). The lab entry is written by Alphys, so she would know the color of the determination she extracted from the souls. While it is not the exact same color as the save point, it is extremely close. Closer than the other yellows found in the game, such as the golden flowers. Coincidentally, Flowey is also the same color yellow as the save point. It’s possible he became this brighter yellow after being injected with determination.
However, the most conclusive evidence is from the Undertale Demo’s instruction pages – specifically the TITLE SCREEN page.
The save point is literally “the manifestation of your own DETERMINATION.” While these instructions are not included in the official release of Undertale, the statement makes sense. Determination is the source of the power to SAVE and CONTINUE/LOAD. It also ties in nicely with the facts that the color yellow is used for selection in the main menu and the save menu turns yellow on a successful save.
Ultimately, there is more evidence that determination is the color yellow rather than red.
Important Note: Please keep in mind that the color of determination is separate from the human soul traits and colors. The yellow soul has the trait Justice, and this analysis does not change that. What this analysis provides is a more appropriate color for the physical component found in all souls – determination – and this disregards it as a trait for the red soul. For those interested in what the red soul’s trait is, please read our theory, the red soul.