The title means "Madder" which is a common plant-based red dye.
The temporary title of this song was "Koharu" (late autumn). I think the both the temporary title and the final title really suits the lyrics, that has a strong red-coloured atmosphere and image of sitting inside while looking at the reddish sunset outside.
This song along with "Horou Kamome wa Doko Made mo", "Iroha" and "8823" was mixed in America, but the sound source was remixed when the band returned to Japan. The sound source that was mixed in America was recorded using an analog board.