Huanglongbing or Citrus Greening caused by Asian Citrus Psyllid and its associated bacterium Candidatus Liberibacter A. causes misshapened fruit and causes the fruit to have a green portion on the skin.  Hence the name citrus greening.  More commonly in Southern California where we have warmer winters than in the Central Valley, without the cold we can often get some green fruit.  It's the cold that brings on the color of fruit.  Especially fruit on the inside of the tree or on the side of the fruit not facing the sun.  Also fruit that hangs on the tree and is over-mature can regreen, losing its yellow or orange color depending on lemon or orange/mandarin.  In this case, we had some fruit brought into the office that was both old and has matured in our warm winter this year.  So the whole tree is covered with fruit that has either green spots or green sides.