What is sunset 23

As long as the branches look healthy and there are some healthy leaves, it should be fine. I have a lime tree in the ground and it is doing the same thing, and has for the last 3 years it's been there. I think it's just the time of year and perhaps getting ready for spring growth (I am already seeing a few blossoms coming in). With that being said, citrus are supposedly high nitrogen feeders, so once all risk of frost is gone (~March in So Cal) I would feed it. That might help it green back up too. You don't want to feed it too soon as new growth is more susceptible to frost if we get a cold snap and not likely to put on too much new growth in winter anyway. Good luck!