The channels are a legal declaration, not a queue choice. Walking through green states formally that you carry nothing requiring declaration; if an inspection then finds otherwise, the walk itself was the offense, which is why penalties for green-channel violations are stiff even when the goods would have been legal to declare.
Use the red channel when any of these apply: you exceed the duty-free allowance for alcohol, tobacco, or overall goods value; you carry cash above the declaration threshold; you have food, plants, or animal products where the country requires declaring them; you carry restricted items like certain medicines or professional equipment; or you simply are not sure. That last one matters, because asking at red costs minutes while guessing wrong at green costs fines.
Expect at red a short conversation, possible bag inspection, and payment of any duty owed. Green is not a free pass either; random stops, X-rays, and dogs operate there. Honest, roughly accurate declarations are treated kindly nearly everywhere, and discovered concealment is not.