Hi!
Nobody seems to have answered your question by now. I'll try to help you.

The first one is correct, "kolikko on hopeaa" - it could also be "kolikko on rautaa", if the fact is that the coin is made out of iron.
- In the same way you could say "Auto on rautaa".

The second sentence doesn't make sense. "Auto on nopea" makes sense, since it expresses a certain limited quality. "Auto on hopeaa" is correct, but usually not the case. "Auto on hopean värinen" describeshow the car looks like in most cases.

"Kolikko on arvokas", "Kolikko on pyöreä" make sense in the same manner.

hopea = silver
nopea = fast

Any help?