It is when we see that internal surfaces of a body are red as usual (or slightly pink), so they are not on of the unusual color (see list below).
It means seeing red : interior of mouth, internal organs, pussy, anus, inside of nose, and other inside surfaces of body.
No requirement for internal view.

Related tags

red_mouth, red_pussy, red_anus, red_internal.

Inside surfaces colors

• black_insides
• purple_insides
• blue_insides
• green_insides
• yellow_insides
• orange_insides
• red_insides
• pink_insides
• white_insides

Do not Extrapolate

Seeing just a red_tongue without seeing that the rest of the mouth (or any other internal surface) is red is not enough for red_insides.
Same for just seeing a red_body and not more.
When the transition between outside and inside is not the same color than the inside color, the <color>_insides tag should be from the color of inside and not from the transition.