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
Idaho requires white lamp on the front, visible from a distance of at least 500 feet. For the rear, red reflector on the rear of a type approved by the idaho transportation department, visible from a distance of 300 feet to the rear when directly in front of lawful upper-beam headlamps. a red rear lamp visible from 500 feet may be used in addition to the reflector. The rule applies whenever it is dark. See Idaho Code § 49-723 (Bicycle equipment — lamps and reflectors).
The cheapest way to stay legal in Idaho 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.