A dark egg layer and a blue or green egg layer. These are some common combinations:
Black Copper Marans x Blue Ameraucana
Cuckoo Marans x Easter Egger
Welsummer x Easter Egger
Barnevelder x Wheaten Ameraucana
Barnevelders are almost never a good choice though as they rarely ever lay dark eggs, in my opinion Blue and Cuckoo Marans are right in that alley too, but Black Copper Marans, Welsummers, Wheaten Marans, Blue Wheaten Marans, and the sort are some good dark egg laying choices. For the blue side, Easter Eggers, Ameraucanas, and Araucanas are all excellent choices.
The really dark olive eggs come from crossing the resulting "Olive Egger" back to a dark egg layer such as a Marans.