If you see a red a green and white light in another boat what does this tell you?

The red (port) and green (starboard) are running lights and can tell you which direction the boat is heading in low visibility conditions. Remember the phrase, "red right returning," which means if the red light is on the right, the boat is returning, or headed towards you. The white light can be a masthead light or a sternlight, and its requirements vary depending on boat length and whether you're operating under international or inland rules.