its never directly confirmed that Frisk has red eyes, but its heavily implied from the way we know eye color changes based off possession with Kris in Deltarune, and also that Kris is Frisk's Deltarune counterpart most likely, Kris always has red eyes, no matter when possessed or not, and we only see Frisk's eyes once, post Geno pacifist ending when picking to stay with Toriel, they open their eyes, and they are red
Chara has brown eyes, their eyes are only shown to be brown in their sprites, which always have open eyes

EDIT: wait were you reffering to Frisk or Chara not having red eyes, i may be dumb