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Yellow Eggplant Fruit
Most people think of large, waxy, purple fruits when eggplant comes to mind. Although many eggplants are purple, not every variety produces this iconic fruit color. Eggplant fruits can range in color from pale green to a deep purple that looks black, including many that appear in shades of yellow, or even white. If you’ve never grown a specific variety before, yellow could simply be the color of the fruit on your plant. Light-colored eggplants are prone to yellowing when they’re approaching an over-ripe state. If this color is appearing on your larger eggplants, but sparing the smaller ones, try harvesting the fruits earlier. Another common cause of yellowing of eggplants is sunburn, which happens when leaves are damaged or removed, exposing the skin of tender, young fruits to excessive ultraviolet radiation. This damage may appear as cream to tan spots, or might cover the entire exposed surface of the fruit. ...
Read more at Gardening Know How: Eggplant Turning Yellow: What To Do For An Eggplant With Yellow Leaves Or Fruit
https://www.gardeningknowhow.com/edible/vegetables/eggplant/eggplants-turning-yellow.htm
Also here https://www.ehow.com/facts_6330591_fruit-eggplant-turning-yellow_.html
Some say it's their mature colour!