Chara's irises are bright red, and Frisk's black.

Drifter's eyes are just all black, sclera and everything.

Chisk has pink irises and I've shamelessly stolen snap's headcanon of grey-blue for Asriel

I think that's all the muses I've ever had

Glorious pink eyes.

it suits them : D
so many pinkeye jokes

(i have a fun time comparing Frisk's eyes to various stones when they're in Determination Mode)

(flint, chert, obsidian, dark granite)

(i'm that kind of writer, yep)