Planetariums (and astronomers, and submarines, and airport control towers) use red lights when it's dark because red light doesn't affect your night vision as much as other colors.
Your eyes have two sets of nerves which sense light - the rods and the cones. Cones can detect color, but the rods are more sensitive to light. That's why you don't see much color in the dark. The rods are not as sensitive to red light, so if you use a red flashlight at night the rods won't adapt to it as much as they would to white light. When you turn the flashlight off, it won't take as long for your eyes to adapt to the dark again.
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