Immediately after a wage agreement is struck, a game of deception and genetic manipulation begins. Laban does not wait a single moment, acting swiftly to tilt the odds in his favor and ensure that Jacob will be left with almost no earnings. Although it is not initially stated who performs the task, the primary approach among commentators is that Laban himself takes charge of separating the flocks [רש״י, רשב״ם, רד״ק, גור אריה ועוד]. He does this on that very same day, stepping in before Jacob even has the chance to approach the animals and perform the separation as they had originally agreed [ביאור יש״ר, מלבי״ם]. Laban’s swift intervention is a direct violation of their arrangement, deliberately designed to eliminate any chance of natural breeding that would produce animals in the colors promised to Jacob.
Laban begins by removing the male goats [רש״י, ביאור יש״ר, שטיינזלץ]. While Jacob only intended to separate the females, Laban fears that the spotted males might mate with the remaining flock and produce offspring that look like them, so he removes the males as well [רמב״ן, הטור הארוך, צאינה וראינה, חומש קה״ת]. Laban is meticulous, removing any animal with an unusual appearance. He separates the goats that have spots or stripes on their legs—specifically in the places where they are typically tied during shearing [רד״ק, רמב״ן, שטיינזלץ], while others describe these as animals with long, wrapping black patches [רלב״ג]. Fearing that even the slightest bit of white could lead to spotted offspring, Laban removes any animal with so much as a single white spot or a tiny patch of white wool [רש״י, רד״ק, העמק דבר, גור אריה]. At the same time, he takes away all the dark, brown, and black sheep [אבן עזרא, הדר זקנים].
As a result of this strict filtering process, Laban leaves Jacob with a flock consisting entirely of pure black goats and pure white sheep. From a biological standpoint, Laban calculates that producing spotted or patched offspring from such a uniform group would require an absolute miracle [רש ר הירש, צאינה וראינה, מלבי״ם]. To secure his advantage, Laban hands the separated animals over to his own sons rather than Jacob's [רש״י, רשב״ם, ספורנו, גור אריה]. He moves them into an entirely separate flock to guarantee they will never mix back into Jacob's herd [שטיינזלץ, ביאור יש״ר]. Yet, despite this extreme cunning, Laban does not dismiss Jacob from his service. He knows perfectly well that his flock is blessed solely because of Jacob. Therefore, Laban leaves Jacob to tend the remaining pure flock so the blessing will continue, while entrusting the separated, uniquely colored animals to the safekeeping of his own sons [בכור שור].