Lights - what you need and when
Between sunset and sunrise, your bicycle must have a white or yellow front light and a red rear light. Both must be active lights - not just reflectors. A rear reflector alone is not sufficient after dark. Each missing light can get you a fine of around €55.
Reflectors are a separate requirement. Your bicycle must have a red rear reflector fitted at all times - day and night. Reflectors are considered permanent equipment, not something you only need after dark. Pedal reflectors and wheel reflectors are also required, though enforcement of these is less strict than the front and rear lights.
In practice, the most common fine cyclists receive is for missing rear lights after dark. Clip-on USB-rechargeable lights are cheap and widely available at any Dutch bike shop - there's no reason to ride without them.