The lightsabers, as large as they are and whatnot, were very noticeable when they changed from green to blue to purple from shot to shot.
The buttons on Vader's chestplate, however, I never noticed or really looked at that closely.
But for all we know, it could change color depending on Vader's feelings. That's how the Emperor really knows what he's up to.
Hmmm... the button is green again... "Are you sure your feelings on this matter are clear, Lord Vader?"