Yellow leaves could indicate a number of different causes, including  iron chlorosis, watering issues (too much or too little), lack of winter watering, etc.  A picture of your tree would be helpful to attempt to give you a more definitive answer. If your tree has been healthy and green in previous years, and now has yellow leaves, it may indicate tree stress due to the very dry winter we recently experienced. You may try an application of chelated iron to correct an iron deficiency.  Applying according to package directions is important and may require repeat applications.  I am including a PlantTalk fact sheet on Iron Chlorosis https://planttalk.colostate.edu/topics/weeds-cultural-problems/2115-iron-chlorosis/
and another CSU Fact Sheet on Fall and Winter Watering 
https://extension.colostate.edu/topic-areas/yard-garden/fall-and-winter-watering-7-211/   to assist you with this problem.