Yellow Aubergine

Has anyone any idea why my Melanza Violetta di Firenze aubergine has turned yellow? It started off purple at the bottom and white at the top, grown in my greenhouse but when we got cold nights I brought it indoors and put it on the windowsill and its gone yellow. Never grown one before so no idea. Its still very firm and shiny.

I think it may have got too cold and won't ripen. Aubergine's like warmth (lots of warmth) and water. However, I could be wrong. When I've grown them they start like yours then get purpler and darker. But one strange thing that I've read is that wild aubergines in warm countries have yellow bitter fruits, but not nice to eat.