Two LEDs, two wires

An LED only conducts one way, so two LEDs wired back to back between the same two pins can be lit separately — this is the trick behind bi-colour LEDs.

Wire, each LED with its own 330 Ω resistor:

• GPIO17 → resistor → red LED → GPIO27
• GPIO27 → resistor → green LED → GPIO17

(no connection to GND). Then loop every 1.5 s:

time	GPIO17	GPIO27	result
0–0.5 s	high	low	red on
0.5–1.0 s	low	high	green on
1.0–1.5 s	high	high	both off

The "off" phase must use both pins high — no voltage difference, no current.