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
Delaware requires white lamp on the front, visible from a distance of at least 500 feet. For the rear, red reflector on the rear visible from a distance of 600 feet when directly in front of lawful upper-beam headlamps. a red rear lamp visible from 500 feet may be used in addition to (but not in lieu of) the reflector. The rule applies whenever it is dark. See 21 Del. Code § 4196 (Bicycle equipment — lamps and reflectors).
The cheapest way to stay legal in Delaware 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.