Q.Aubergines
Why is the fruit yellow and not purple?

Eggplants do come in many colors other then the traditional common purple color.
They can be from white, cream, green to even near black.
There can also be issues with over ripeness, nutrition or insects that may cause yellowing of the Eggplants.
Here is some links to help you.
https://www.gardeningknowhow.com/edible/vegetables/eggplant/eggplants-turning-yellow.htm
https://www.gardeningknowhow.com/edible/vegetables/eggplant/growing-eggplant.htm