Hi, Eric Rasmussen here. I can't recall why I decided to say the 1987 512Ke was platinum. I'm usually careful about these things, and try not to make assertions for which I have no proof, but sometimes the temptation is too great. Anyhow, here's what I found in my archives.
There is the famous label, which appears to be platinum, but it is a photograph so not reliable [Note: I have not adjusted the color in any of these photos, which are mostly from eBay]:
On the boxes, the 512Ke got a new box in 1987 like every other machine in the line (i.e., the Plus and the 512K/800 in Europe). Here is a photo (from the same auction as the label):
I don't know what to make of the image. I agree with Mac128 that it looks somewhat beige (compare to the 1987 Plus box, below), but it is not conclusive by any means.
Here is the 1986 512Ke box. Notice the photo is different, from an angle:
The same change occurred in the Plus boxes. 1987 Plus box:
1986 Plus box:
I wish now I had just bought that damned 1987 512Ke...