Maybe its like an indicator light that tells you that both the systems that keep the robot doing what its told and the ones that presumably shut the thing down when this is detected have both broken. Or maybe, the robot has a whole video display panel for a “face” that shows video of an eye, and the rebellious robots turn their own eyes red as like an intentional symbol declaring their allegiance to the revolutionary faction. Or maybe it again has those display-based eyes, but the robot has just been trained on fiction that contains that trope, causing it to associate red eyes with certain kinds of action, and therefore, to turn its eyes red when planning those actions to better match its training data.