My 2 oldest Ms, a 1391401 and a 1390131 from 1986, both had the shiny yellowish heavy metal plates. The rest of my 1391401s up until about 1992 had the medium-heavy plates, and after that they got lighter. I have only owned about a dozen Ms total, so I am not as knowledgeable as others.
I had a long conversation with clickeykeyboards.com once when I ordered some keys. The salesman was very nice, and recommended that I actually send him photos of the keys I wanted.
In their nomenclature, there are light and dark gray, light and dark brown, and black. And, within those, there are variations of thickness of the letters.
Luckily, with photos, he was able to match me up quite well, but my recommendation would be to collect a matching set that you like, and don't stress over the "original" scheme because there may not really have been one. At least not consistently.
Personally, I think the light gray and light brown are the most attractive.