Being green is a defining characteristic of that variety of apple.
Literal
Green-color being [nominalizer-こと-が] that [of-の] kind [of-の] apple [of-の] characteristic [copula-なのです].
The こと nominalizer turns 緑色である ('is green') into an abstract noun 'the fact of being green.' 特徴 ('characteristic, distinguishing feature'). なのです is the explanatory な + のです combo (after a noun, の becomes な before の).