Normal trichromats read Ishihara plates using their L- and M-cones. Red-green color vision defective observers rely on S-cone and luminance signals in reading the plates. Some normal trichromats can read the hidden digit plates because they can extract S-cone signal difference efficiently despite the distraction from L- and M-cones.
https://jov.arvojournals.org/article.aspx?articleid=2135713
Normal trichromats read Ishihara plates using their L- and M-cones. Red-green color vision defective observers rely on S-cone and luminance signals in reading the plates. Some normal trichromats can read the hidden digit plates because they can extract S-cone signal difference efficiently despite the distraction from L- and M-cones.