Laban is concerned about Jacob’s request to leave. He knows that he has prospered much as a result of Jacob’s expert handling of his livestock, but there is no reason why Jacob should stay any longer, given the terms they have agreed in the past, for Jacob has now completed his fourteen years of service for his two wives: seven for Leah, and seven for Rachel.
Was Jacob still acting deceitfully when he proposed these terms, so that it was one cunning man doing battle with another cunning man? No, that is to misread his character, and to ignore his encounter with the Lord at Bethel. He had vowed that the Lord would be his God (Genesis 28:21) and that meant that he abandoned his former ways. What follows should not be read as if Jacob was using some natural means of controlling the colour of the offspring. This was entirely of the Lord, who overruled on Jacob’s behalf. We need make no attempt to explain this in scientific terms as if there is some physical or genetic reason for what took place. God was instructing Jacob to carry out this procedure. The Lord was going enrich Jacob at Laban’s expense and we are not told how he did it.