dark red
綪 centers on a specific shade of dark red, often associated with silk or fabric contexts.
綪 combines 糸 (thread/silk) with 青, which likely contributes the sound and possibly the color association. The exact historical development is uncertain, but the character likely originally referred to a dark red silk fabric.
The thread radical 糸 on the left and 青 (blue/green) on the right suggest a fabric dyed a deep, dark red—a color that might be produced by overdyeing with indigo and madder. Picture a bolt of silk in a rich, dark crimson.
For セン, imagine a bolt of dark red silk costing one hundred (百, ひゃく) sen (銭, せん) coins: sen -> セン, and the fabric is worth every coin.