Simply red

Hibiki Kurenai makes his debut in Kamen Rider Hibiki episode 24. “Kurenai,” of course, means “third-degree burns.”

Okay, it actually means “crimson” and I imagine it’s thus named because the Bandai/Plex designer spilled a bottle of crimson paint all over his design plan and then decided to pass it off as Hibiki’s new power-up form.