HSL
HSL stands for Hue, Saturation and Luminance. The HSL panel lets you control each color separately, so you can adjust just the blue of the sky without touching anything else. It's the heart of targeted color work.
Want green to lean a little more yellow-green, or skin to be slightly less red? With HSL you do that per color, independently. Hue shifts the color, Saturation makes it fuller or duller, Luminance makes it lighter or darker. A common trick: lower the luminance of blue for a deeper sky. Use the targeted adjustment tool to drag right on the photo, and the software picks the right color automatically.
Read also the in-depth explanation: HSL 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.