noun
rusty color from faded black or purple clothes
Describes the reddish-brown tint that appears when black or dark purple fabric fades, resembling the color of yōkan (sweet bean jelly).
長年着た黒い着物が羊羹色になっていた。
The black kimono I wore for years had turned a rusty yōkan color.
羊羹 is the sweet bean jelly itself; 羊羹色 refers specifically to the faded fabric color named after it.
From 羊羹 (yōkan, a dark reddish-brown sweet bean jelly) + 色 (color). The term arose from the resemblance of faded black or purple fabric to the color of yōkan.