My first thought would be that they need Nitrogen. I don't know what type lemon you have, but I can tell you that Meyers lemons are a very heavy nitrogen feeder, mine always needs supplimental nitrogen to keep the leaves green. Also lemon leaves tend to not be as green as other varieties of citrus at the best of times. If you have calcareous soil (which is common to your region) and want to give them a quick shot of nitrogen to boost the green I would use Ammonium sulfate in moderation. Something like this https://smile.amazon.com/Ammonium-Sulfate-Easy-Peasy-Plants/dp/B07BTPJCT5  alternatively if you grow organically, you could use blood meal as a fast acting form of nitrogen. https://smile.amazon.com/Espoma-DB3-Dried-Blood-Pound/dp/B001JTAJFM  though you would need to use about 1/3 more

Thank you!