One pin, one light

Three loops that share one wire

Pick which pins are HIGH. Current flows round those loops and no others. Each one runs from your board's pin, through an LED, through that LED's own resistor, into GND and back to your board. The net list has exactly those three paths and nothing else: no part connects one colour to another.

GND is the only wire the three share, so it is the only place their currents add up. The rest of the time they are three copies of the XL LED's circuit side by side. Where the current goes follows one loop in detail.

One resistor each

Each LED has its own 1 kΩ resistor, printed 1001. One resistor for all three, on the shared GND wire, would not work: the lights would share its current, and each one would get dimmer as the others came on. With one each, every light's current depends only on its own pin.

The resistor is what keeps the current safe for your board's pin, and what the resistor is for explains why an LED cannot do that for itself. 1 kΩ is a generous value. It also makes the arithmetic easy: every volt left over after the LED is one milliamp.

Nothing stops all three

A real traffic light's controller is built so that conflicting lights cannot show together. This board has no such thing. It is three LEDs and three resistors, and it will light green with red if both pins are HIGH.

So the sequence lives entirely in your sketch, and a sketch that turns one light on without turning the others off will show two at once. The sketches in this book avoid that by setting all three pins in every step, even the ones that do not change.