Always busy at work, your citrus trees in winter are usually holding and ripening up a crop of citrus fruit!
This requires energy! They are also snugging down and locking in for the winter months - This also requires energy.
Slightly yellow (not seriously yellow) leaves during winter is nothing to worry about, your tree is redirecting energy and preparing for the colder nights ahead.
Best not to feed your Citrus Plants in winter leave them to hibernate and do their winter work.
As soon as the weather warms up, you may commence your feeding regime.