Answer: That is white latex paint designed specifically for this purpose. Citrus bark is vulnerable to sunburn damage, which appears as discolored tissue that may crack and peel, leaving an easy entry point for pests and diseases. The paint protects the bark, especially on southern and western exposures. However, if you do not prune the trees up, and leave the low branches as a "skirt" to protect the trunk, it doesn't need to be painted. This is actually a healthier way for the tree, and easier to pick the fruit, but most people seem to want to trim them up radically, just because everyone else does it that way!
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