I always thought it was silver when I was a kid. Even painted one AMT silver, which was a very "soft" (mushy) testors color, if you know what I mean.
I left mine white and it looked okay, but not just right.
My first Klingon battlecruiser came molded in gray plastic and it looked right on. I loved that: it was just like the aired version. My second AMT Klingon ship, to my shock, was molded in black plastic. That was just weird.