What does it mean when you see only a red and green light on another boat?

If you see both the red and green lights but not the white one, the boat is coming right for you and something's wrong because the white light has to be visible from any angle for a distance of two miles.

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If there's no white light it's a sailing boat. If you see red and green the boat is under sail and you are lined up with its bows. A small motor boat also needs a white light high up and visible all round.