Headcanons for the Humans and the goats eyes! Explanations under cut!
Chara: We can see in the game that their eyes are brown, specifically darker shade of brown than their hair! Their eyes will glow red in reaction to the red human SOUL, however. I believe their eyes being rendered brown proves that there is a "real" eye color beneath the color of the SOUL/magic!
Susie further convinces me that the eyes 'flicker' red rather than just being literally red, as another example of someone with the known eye color having eyes glow red while using the red magic!
Kris and Frisk: I do think that they should look identical with the exception for their age! The only thing I've changed is their eye colors, giving Frisk the amber ("golden") eyes and Kris grey ("silver") eyes! It seems thematically appropriate as it is associated with the golden Determination vs silver 'shining light' respectively. I kept them to realistic human eyes tone, however Frisk's magic also shows the gold!
Unused sprite of Frisk renders their skin tan, or simply light brown, which I believe was the intention for Frisk's appearance before Toby decided to omit their race and make them "like Simpsons"! I just use this skin tone for them, except Kris is just a bit paler as someone not hanging out outside as often as Frisk!
Toriel and Asgore: I believe that Ralsei is the most crucial insight on this species of Monsters! He has pink horn, hair with pink overtone and pink eyes! Using this logic, Asgore, who has golden hair and horns with golden overtone would have golden eyes and Toriel would have light grey or white eyes as someone just white without any overtones!
I believe that Toriel's red eyes are not a coloring mistake as the red persists even when she blinks and isn't just in a single sprite, however this is gone in Deltarune and doesn't appear in any other official portrayal of her! This leads me to believe it is also the red magic. It especially tracks since the nameless red trait is inherently angelic and she is a very religious person. Contrary to the common idea, cyan 'Patience' or the orange 'Bravery' magic that Asgore uses are not attached to any particular eye but interchange randomly just to communicate which type of the attack will come from which side! Here ( x ) is a video with an example of one 'Patience' attack from the left and three 'Bravery' attacks from right, left and right again!
Asriel: Using cartoon genetics logic, his eyes would be a mix of grey and golden! The thing is, the 'organic' (not the 'screen'!) eyes of Omega Flowey seem to be EXACTLY this color:
It is very greyish yellowish-orange color! Another possible pointer towards his eyes being something like this is what color you get by tweaking Ralsei's sprite! Asriel's outline is blue and Ralsei's outline is green, and when you tweak Ralsei's colors into changing green to blue it also changes his pink to desaturated yellow! And, of course, there is the "strawberry nerd and vanilla nerd" joke!
I simply used the 'screen' eyes as the red trait magic and green 'Kindness' magic. It seems appropriate as Asriel is angelic and he certainly used to be kind whether he believes it or not. I like to think he at least healed Chara's broken bones when finding them. Ralsei, who is connected with him in some way, is also a cleric; both angelic and a healing magic user!
Bonus: I believe Togore would then work the best with one grey and one golden eye, to use the reverse cartoon genetics with Asriel. And since I've made Asgore who is golden sun-coded, his father could be red and fire-coded! Beta Asgore even DID have red hair at some point! I am now using this but these ideas feel like they're for the different kind of fanart entirely :')
