Well, it could be a couple of variables causing your problem....
Lack of sunlight, overwatering, lack of iron which they highly require, lack of nitrogen also a high need, even temps.
Knock off possible cultural problems before you even worry about fertilzer. They won't appreciate a good fertilzer if the conditions for optimal growth are not there. Then move forward with this....
This is a fact....Citrus REQUIRE much more fertilzer than most plants. Especially nitrogen and iron. Why even calcium.
If you want to know what is causing your leaves to yellow, first make sure your not overwatering causing the roots to not take up nutrients correctly.
Then if it is nitrogen defficient, I would feed an acidic fertilizer for acid loving plants.
Then if this is not making a difference, definately try chelated iron! This is what was wrong with mine.
Without seeing pics of your trees or leaves, I am just assuming it has to be caused by any of these issues. These are common causes.
If you google "yellowing leaves on citrus", you will find a site that can match pics of your leaves to the right problem.
Try these, and let us know what happens.

1. Make sure you are not over watering.
2. Feed an acidic fertilizer right away.
3. Try chelated iron also asap if you just want to fix your yellowing with out knowing which one did the trick.... I mix in the chelated iron with the fertilzer together. It works for me.

Once you have them as green as you like, then use a regular small dose of the best ferilzer I have ever used called, Foiliage Pro at every watering.
It has every essential nutrient in it including calcium plus the perfect ratio of N.P.K!
This stuff you should use to maitain the health of your trees..
By the way, are they in containers?...MIne are containers. I am not a pro at in ground. They must have citrus fertilzers for inground over the webb? But even inground citrus have a high need of nitrogen, iron, and calcium, plus all the minors!
Hope this helps you. You can green them up fast. I have done it..:-)
Mike

If you post a picture of the leaves then someone could say what defficency is there, they yellow in different places for different things, but often the deficency is lack of acidity causing lack of nutrient take up not a real defficency,

as above, adding acidic fertilisers and trace element blends often cures it, but for some deficencys spraying the leaves works better, things like magnesium and manganese