considering purchasing cold start valve for 1976 datsun 280 z from auto parts store, one valve has a black marking, other valve has a green marking, price difference is $9 could any one tell me if there is difference, both from same manufactor

Many manufacturer's offer a cheaper part but give a shorter warranty. That is probably the main difference.

The cold start valve's only purpose is to squirt some extra gasoline in to the intake manifold when the key is at Start and the engine is cold. Many people remove theirs to remove one potential problem. I've removed mine and don't even notice it's gone. I live in Oregon where it rarely gets below freezing though. If you live in Minnesota, you might want to keep it.