Well, the attack damage reds/marks are for early game harass/last hitting. You gain an additional 15 attack damage. Which is basically a long sword/dorans. And most adcs won't go cloth armor or any armor for that matter. So armor penetration is unnecessary. :)

Nice build, xGenesis! However, i was wondering why did you use the attack damage marks instead or armor reduction... Do they scale better with Tristana?