The craftsman dyed the cloth scarlet.
The sunset dyed the sky crimson.
The sky turned a deep scarlet with the sunset.
緋 refers to a vivid scarlet often used for traditional textiles and armor, while 朱 leans more orange and is associated with ink and seals.
緋 (scarlet) and 火 (fire) are pronounced identically as ひ but are completely unrelated in meaning, distinguished only by kanji and context.
scarlet; cardinal red
deep scarlet
Taiwan cherry; winter cherry tree
scarlet lacing; scarlet bindings · scarlet-laced armor; armor with scarlet lacing
red hakama
red carp; golden carp
scarlet; bright red
scarlet robe
Taiwan cherry; bellflower cherry
The kanji 緋 is composed of 糸 (thread) and 非 (a phonetic component), suggesting a connection to dyed thread. The exact historical derivation is uncertain.
hiboke flowering quince; Chaenomeles lagenaria
himedaka; gold-colored Japanese rice fish