虾红素

• 虫(insect; small animal)RADICAL
• 下(below; under)PHONETIC

A shrimp is a creature that crawls along the bottom of the water.

• 糹(thread; silk fiber)RADICAL
• 工(craftsman; work)PHONO-SEMANTIC

A thread dyed by the craftsman becomes bright red.

• 糸(silk thread; filament)RADICAL
• 龶(coarse undyed thread bundle; top part of thread)ASSOCIATIVE

Raw silk that hasn't been dyed suggests the meaning of pure, simple, and basic.