The easiest way to tell the true color is usually to look at the feets of the drive, it tends to be the one place that doesn't chance color. The platinum models had platinum/gray feets, the beige ones had beige feets.

Also, as far as I'm aware, from what I have read on the subject, you cannot use an M0131 external drive on a 512k with stock ROMs unless it has already loaded the HD-20 init. It's a deficiency they apparently corrected with the subsequent model.