As the Samaritan episode unfolds, Jesus tells the disciples to lift their eyes and look at the fields, 'for they are white already to harvest.' The metaphor comes from an agricultural landscape familiar to Galilean and Samaritan hearers.
Harvest language turns the disciples' attention from food and timing toward the people around them.
Grain agriculture was central to the economy of ancient Palestine. Sowing, waiting, ripening, reaping, and gathering provided ready images for time, labor, and reward. Jesus continues with sayings about reaper, wages, fruit, and the distinction between sowing and harvesting.
Attempts to prove that a particular crop literally appeared white at that exact moment go beyond John's statement; the force of the image does not depend on identifying a specific field.
The agricultural image interprets what is happening in Samaria. The disciples might regard the region primarily as territory to pass through, but Jesus sees people ready for gathering.
The following response of the Samaritan townspeople gives narrative substance to the metaphor: many believe, approach Jesus, and ask him to remain with them.