Unlike some garden vegetables, cucumbers can be a bit of a puzzle. Cucumbers don’t give as many obvious cues to a novice gardener that they’re ready to pick, and a great many ailments of the cucumber plant present similar symptoms.
If your cucumbers are turning yellow, but the fruit is large and it and the plant itself seems otherwise healthy, the chances are that your cucumber is becoming overripe. You should pick the cucumber right away, and pick future fruit just a little sooner. A fully-ripe cucumber is firm, and bright medium green to dark green. As they turn yellow, cucumbers begin to turn bitter. You can salvage a cucumber with a hint of yellow in it, but may find that the end near the stem is tastes unpleasant and has to be discarded.
(Large) yellow cucumbers and an ailing plant, or sometimes green but oddly shaped or shrivelled cucumbers, are frequently signs of a nutrient deficiency. Frequently, it’s a lack of the major nutrients. Imbalances in nitrogen, phosphorus, potassium and/or magnesium all present similarly, and can be caused by over-watering (resulting in washing out of your fertilizer). Stunting may also occur in the case of a nitrogen imbalance. They usually cause the plant’s oldest leaves to yellow, burn and die off in slightly different ways. In most cases the treatment is the same: a more regular application of a balanced fertilizer (with micro-nutrients in particular, if you’re using a soil-less mix) to help meet the plant’s needs and see if this fixes the problem.
Additional calcium may also be required by the cucumber plant, especially if your garden has a history of ailments from calcium deficiency (ie blossom rot). Frequently, in the case of calcium deficiency, the plants appear healthy, but the fruit yellows and drops long before maturity.
Pests can also cause your cucumber plants some grief, though it should be easier to diagnose. You can cut open and examine dying fruit for worm intrusion, or examine under your leaves for the webbing typically left by spider mites.
Diseases and viruses can also cause the yellowing of cucumbers and the leaves, and may be the culprit if new leaves are being affected, or isolated “sick” spots are found. Plants affected by viruses should be pulled and destroyed.
Lastly, and amusingly! I read online a story about a woman who got only yellow, bitter cucumbers from her garden. They never became green at all. It turned out that the cucumbers were planted too close to some squash plants, and cross-pollination occurred between the two species. So plant your squash and cucumber well apart!