This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
Washington requires front lamp emitting a white light visible from at least 500 feet to the front. For the rear, red rear reflector approved by the state patrol, visible from all distances up to 600 feet when directly in front of lawful low-beam headlamps. a red rear lamp visible from 500 feet may be added but cannot replace the reflector. The rule applies whenever it is dark. See RCW 46.61.780.
The cheapest way to stay legal in Washington is a USB-rechargeable front white light and a rear red light or reflector that meets the spec above. A $30 light set covers it.