plant yielding a red dye
茈 refers to a specific plant used to produce a red dye. Its meaning is narrow and concrete, centered on that botanical source of color.
茈 combines the plant radical 艹 with 此, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with a dye plant.
The top 艹 shows it's a plant, and the bottom 此 looks like a container with a stopper. Imagine this plant's roots being crushed in a container to release a vivid red dye.
For シ, picture a sheet of cloth being dipped into the red dye from this plant: sheet -> シ, and the cloth turns red.