RCW 46.37.5185: Street rod and custom vehicles—Blue dot taillights.

A street rod or custom vehicle may use blue dot taillights for stop lamps, rear turning indicator lamps, rear hazard lamps, and rear reflectors. For the purposes of this section, "blue dot taillight" means a red lamp installed in the rear of a motor vehicle containing a blue or purple insert that is not more than one inch in diameter.