madder, red dye, Turkey red
茜 centers on a plant used to produce a deep red dye: madder. The color sense (Turkey red) is a direct application of that dye.
茜 combines 艹 (grass/plant) with 西, which likely contributes a phonetic element. The character refers to the madder plant, whose roots yield a red dye.
The grass radical 艹 sits above 西, like a plant growing in the west. Picture a field of madder plants in the western sun, their roots full of red dye.
For セン, imagine a thousand (千, せん) madder roots being harvested to make red dye: 千 -> セン, and the roots turn the dye vat deep red.