@Bingowings

Yeah the Red Light Never Bothered me.

It could very well be a remnant from the first design choices of the AT-AT’s though because they were also originally going to have Blue Lasers. I think the red lit cockpit certainly from an external point of view makes them look meaner.

It could also have been a simple decision so that in the shot where we see the AT-AT with the Red light on (aside from the Binocular shot) it draws our focus to that particular AT-AT as this is the one that launches the attack and fires the first shots.