darkened balls
Darkened balls describes a character's balls that are, even if not as such very dark in color, noticeably darker than the surrounding skin or fur, without necessarily being a distinctly different color. Hyperpigmentation of the anus and genitals is commonplace in real life. The color of the balls should be a darker hue of the primary or secondary body color.
This is commonly paired with darkened penis.
See also
This tag implicates darkened_genitalia, balls (learn more).