I believe that the subtraction CMYK color model is deeply tied to the meanings of colors in the DELTARUNE
Magenta - Yellow = Red
Cyan - Magenta = Green
Cyan - Yellow = Blue
Cyan - Magenta - Yellow = Black
I assume that every color of CMYK has it's own meaning, and if combined they create new RGB colors with their own meanings
Black definitely stands for Darkness. However, I believe that in this scenario, Black is made out of Red (M-Y) and Cyan.
Red stands for DETERMINATION, "willpower", "will to live". When you create a dark fountain, your eyes turn red. "Thrusting the will into it's blade, it made".
Willpower is made out of Magenta and Yellow. I believe Yellow to be "Power", and Magenta to be passion, "perseverance", "Will" to use this power.
Magenta(Pink) and Yellow appearing separately in the same place symbolizes the strive, "passion" to gain that Power which is out of reach. This is the reason why Spamton has Pink and Yellow eyes.
Friend's eyes are inverted. Instead of Power being unreachable, it's agency, passion and will is out of reach, whilst having all the Power. If you haven't forgot, it's a cat, a humble servant that follows the hand above to the utmost... Spamton's eyes invert when he laughs, but it's only because of his ties to Friend.
Cyan is inverted Red. I assume that it's reverse of DETERMINATION - Nihilism/Fatalism. You give up instead.
Seam's shop is orange, and orange(/cyan are a sort of ying-yang in UT/DR, as if Orange is an inversion of Cyan.
Yellow - Cyan = Blue. Sans is Blue and his left eye turns cyan and yellow. He has all the Power, all the knowledge, but he doesn't use it, as if he gave it all up. Kris is yellow in the Light World, and becomes blue/cyan in the Dark World, because Yellow also symbolizes Freedom and Control, not just "Power".
Magenta - Cyan = Green. Ralsei has all the perseverance to continue, even though he knows that his fate is already decided. If we believe in Greenster, you could say that Gaster gave up his existence to persevere even after death.
But since Darkness (Black) is made out of DETERMINATION and something else that is Cyan, what it could be? It could be Angel's tears. This is supported by the fact that Dess has Cyan eyes in that mantle game, and if it's true that the UNUSED is her, she could be "crying for help". But you could also just call Cyan the color of Water.
If you believe in my schizo "Missing Red Skeleton Brother Theory", this CMYK theory could imply that the RGB skeleton family had an evil Red brother, who has or uses DETERMINATION. and papyrus is vermilion so he doesn't count
