noun
madder plant; Japanese madder; Rubia argyi
Refers to the plant itself, especially the species Rubia argyi, whose roots yield a red dye.
茜の根から赤い染料が採れる。
A red dye can be obtained from the roots of the madder plant.
noun
madder red; deep red
The color produced from madder dye; a deep, slightly brownish red. Often used in traditional Japanese color names.
夕焼けが茜色に染まっている。
The sunset is dyed in madder red.
Specifically the color madder red, while 茜 can refer to the plant or the color.
The origin is uncertain. The kanji 茜 is conventionally associated with the madder plant and its red dye.