The green rose is exactly that -- a rose varietal that is green in color. There are two kinds: the original without petals, and the more modern mutated forms that have actual flower petals in varying shades of green. Beware of those who offer seeds: the original green rose does not produce pollen and cannot produce "seeds". It can only be reproduced by cloning (cuttings).
I first saw these in a comprehensive rose handbook back when I was researching old roses (ie, non-hybrid types).