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
Montana requires lamp on the front emitting a white light visible from a distance of at least 500 feet to the front, required during darkness or whenever low visibility makes it difficult to identify persons and vehicles at 500 feet. For the rear, red reflector on the rear of a type approved by the montana department of justice, visible from all distances from 100 feet to 600 feet when directly in front of lawful lower-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 MCA § 61-9-407 (Lamps and other equipment on bicycles).
The cheapest way to stay legal in Montana 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.