Cucumbers that are yellow are overripe. Cucumbers that are yellow, have sunken areas, are wrinkled on the ends or puffy should be discarded. Cucumbers that are uniformly green and firm are of the highest quality.
Cucumbers are green because of the chlorophyll produced in the plant. Once they become overripe, the green pigment from the chlorophyll begins to fade, leaving behind a yellow pigment. Yellow cucumbers are generally not fit to eat. Cucumbers are ripe and can be harvested typically 50 to 70 days after planting. A ripe cucumber is medium to dark green in color and feels uniformly firm.