Yellow cucumbers are overripe cucumbers. Ripening hormones turn the fruits from green to yellow as they respire.
But some varieties, like the lemon cucumber, are naturally yellow at harvest. The best way is to know the maturity days of your growing variety.
Exposure to high temperatures ripens cucumber fruits quickly and causes yellowing.
Yellow cucumbers may be harvested from nutrient-deficient cucumber plants. The lack of nitrogen used to make chlorophyll causes yellowing in fruits.
Fungal disease and cucumber mosaic virus may cause fruit yellowing, especially when planted in high density.
Yellow cucumbers are safe to eat, but be cautious of soft spots because they can be bacteria-producing toxins.
Also, washing your cucumber fruits thoroughly before eating them is the best practice.