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This can happen for a few reasons, but most likely it is either due to a nutrient deficiency or because the plant is growing in very bright sunlight.

I would give the plant some balanced fertilizer and unless you see any other symptoms, such as yellowing leaves or spots on leaves, don't worry about it. If the fertilizer does not correct it, then it is likely just a defense mechanism that some plants have when they are in very harsh light. New leaves can grow out initially red to protect them from the sun and then as they mature will develop their normal green color.

It may depend on whether or not the tree was seed grown or obtained from nursery stock. Oftentimes, lemons grown from seed may or may not produce fruit, but if it does produce fruit this may take many years, and when it does produce lemons, the lemons MOST LIKELY, will not be edible or extremely slow growing. Since there's really no way of knowing for certain, you could try adding more phosphorus (bone meal) to the soil to help encourage the flowering and fruit growth.

You may want to send in a picture for identification.

I would try giving it some phosphorus rich fertilizer, like bone meal. If it is lacking phosphorus, it will not be able to produce blossoms (which means no fruit.)

The requirements for lemons are very similar to that of limes. If you have not read it yet, this article may also be helpful: https://www.gardeningknowhow.com/edible/fruits/lime/reasons-and-fixes-for-lime-tree-not-producing-blossoms-or-fruit.htm

You need to keep the plant evenly watered at all times. When growing fruit, citrus trees need quite a bit of water. I would not cut back water for them when they are fruiting, especially when they are in containers. In the ground, they can access water from deeper in the soil, but in containers, you are their only water source and it is just best to keep them well supplied for best growth. These articles will help: https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm

The trees may be suffering from an iron deficiency. This commonly causes this kind of coloration on the leaves and normally happens either when the watered too much and the nutrients get leeched away or if the pH is too high. Test the soil and correct and nutrient and pH issues found when testing. Also, make sure that the soil is not too porous and is letting the water run away too quickly. Here is more info: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm