I found this picture of young citrus fruits on the web site of an Italian nursery.
Any idea what these are? From the leaves I would guess some kind of lemon...

http://vivaicavallaro.it/EN-citruses-in-bloom.php

I think, young fruit of citrus limon can develop
this colour, when exposed to full sun.

It is indeed a lemon and when they flower in cool / cold conditions the new growth & small fruit have that dark purple color. I have noticed this with all of my lemons & citrons. In the summer when they set fruit & flush the color is normal green. When the flower & flush this time of year outside they are all purple.

So how cold does it have to be to make them turn purple? The young fruits on my feminello are perfectly green. I have flower buds on both the feminello and the buddhas hand citron, they are all perfectly white flowers with no pink. Temperature currently fluctuates between 12 and 15°C where the plants are stored.

Right now here it drops down into the low 40's F at night & sometimes into the 30's F. All the new growth on my Feminello is purple as are all of my variegated lemons, also the small fruit.