I’ve gotten a couple calls this year about citrus fruit that looks brown on the outside of the rind but the fruit is still good. The likely culprit is the Citrus Rust Mite — a very common pest of citrus. The damage it causes is mainly cosmetic, but renders fruit unsaleable. The mites can be controlled with miticides, but it requires multiple spray applications. Therefore, sprays should be limited to those trees that will be producing commercial fruit. Homeowners can frown at the outside but delight in the inside. See photo below for a visual (ignore the shriveled fruit). This photo came from Tracy Boyd, Harrison County Extension.