In all the OT films, if you can study production photos, you can see the faceplate metallic gray and the chest armor's metallic gray are the same color. There are times where people have thought differently and use the ROTJ Death Star landing bay scene as the basis of why Vader is supposedly very dark, or why the armor is brighter.
https://www.facebook.com/notes/sculptin ... 788285385/This article discusses a set photo, and you can see there is plenty of black on the floor and walls (offscreen) that reflected on the mask, making it look dark. Also, the publicity shot of him in the landing bay looking right into the camera - the armor is reflecting a lot of overhead lighting based on its angle in relation to the face.
Here is an ESB production photo.
http://www11.pic-upload.de/16.12.14/xmebi7xebcc5.jpgAnd ANH:
https://qph.ec.quoracdn.net/main-qimg-f ... 16148cef-cIn each of the above examples, you can see the metallic gray paint of the face and armor are within the same brightness range when they are not terribly contrasted by set lighting.
Hope this helps!