I have a yellow eggplant on a plant in my back yard--I assumed it was needing water and fertilizer, which it hasn't had enough of. I've tossed a couple of others in the compost--they were yellow, but also soft. The same plant has a purple fruit also, from a later flower and after it has had some water.

It might taste odd, but I haven't tested it. I think this isn't that unusual if you've had a bad year for other stuff (which we've had here).

SRS