I think what bothers me is that "plain", "metallic" and "translucent" are mainly meant to describe plastic colors, whereas "greyscale" and "full color" are printing terms. When we snuck "full color" into the "plain" material it was as an afterthought, but we knew it didn't really make sense when talking about plastic (we mention as much in the widget help text). We got away with extending the idea for prismatic/holographic and other shiny printing effects because if you use your imagination you can think of these things as being "metallic", but really we were stretching it.
Now that 'ol Spliffy is introducing the idea of "greyscale" (and even greyscale + red), we really need to re-evaluate the whole idea.
It's been 20 years since my graphic design days, but if memory serves "greyscale" is really just another term for monochromatic printing with black ink. The greyscale + red sheets mentioned by Engledogg are technically just duotone, meaning black and red ink.
So I guess what I'm suggesting--for the moment, anyway--is that we just completely separate the plastic colors from printed colors. We would add a "Printed" material (alongside "plain", "metallic", and "translucent", which would be understood to refer to plastic colors), and it would be home to printing terminology. It would list the same colors as the other boxes, but also have "Full color" and "Prismatic/Hologram" and other fancy effects. To describe the greyscale/red item mentioned previously, you would just check "Black" and "Red" (but not "white", as no white ink is used).
I have to head off to see the family in a few, but I'll see if I can produce a comp before they come pick me up...