even modern copper plated zinc coins with the smallest amount of copper plating are graded rd, Rb or br.
That's because copper-plated coins are still copper-coloured and, since they have a copper surface that's chemically identical to the surface of a solid copper coin, they have exactly the same colour transition, so the same labels can be applied.
Even with silver why not add toned to the lable?
I suspect they don't mention toning because toning on silver is, like on brass, much harder to quantify. Silver tones black, which is almost universally regarded as "bad". But the opinions on the colours in between untoned ("white") and black vary widely. And, as with brass, there's a whole spectrum to account for, not to mention the different combinations of colours that can occur on the same coin. Just a simple "Toned" adjective isn't going to cut it. And it would be hard to put "red-gold toning on the obverse, blue-green on the reverse with occasional black spots" all on the label.
One of the advantages of including a colour on-label, means a sight-unseen price guide can be fixed. People universally agree on the price scale for coppers: red>>red-brown>brown, so the price guides can reflect this. But apart from "black is bad", there is no such universal opinion on the effect of toning for silver toned coins, because the broad spectrum of possible colours means people can have genuinely different opinions, which cannot be reflected in a price guide. You might like red-toned silver, I might like green, and the guy who writes the price guide might think blue is best. Why should you and I have to pay more, for coins that we actually like less, just because the price guide guy says so? And what about the people who dislike, and therefore require a discount for, toning of any kind? Their opinion about toning isn't "wrong", it's just "different". And the concept of a single unifying price guide cannot survive such differences of opinion.
How to handle toning, and the impact that toning can have on eye appeal and thus on perceived value, is one of the major drawbacks to the TPG
"sight-unseen-sales" business model. The red-rb-brown scale attempt at quantifying this works well for copper, but wouldn't work for other metals.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis