Jacob separated those lambs from Laban’s flock. And at mating time he turned the flock to face Laban’s animals that were streaked or black. This is how he built his own flock instead of increasing Laban’s.
“For twenty years I have been with you, caring for your flocks. In all that time your sheep and goats never miscarried. In all those years I never used a single ram of yours for food.
In fact, if the God of my father had not been on my side—the God of Abraham and the fearsome God of Isaac—you would have sent me away empty-handed. But God has seen your abuse and my hard work. That is why he appeared to you last night and rebuked you!”
For if he said, ‘The speckled animals will be your wages,’ the whole flock began to produce speckled young. And when he changed his mind and said, ‘The striped animals will be your wages,’ then the whole flock produced striped young.