Defringe
Defringe removes the colored edges (purple, green or magenta) that sometimes appear along sharp contrast transitions, a form of chromatic aberration. You see them mostly in backlight and at the edges of branches or roofs. The defringe sliders take out those color fringes precisely.
Start with the 'Remove Chromatic Aberration' checkbox, which fixes most fringes automatically. If a purple or green edge remains, use the eyedropper in the defringe section and click on the colored edge itself. Then control the range with the amount and hue sliders. Go easy, because defringing too aggressively can affect real colors in your photo.
Read also the in-depth explanation: Defringe explained.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.