Split toning
Split toning lays one color over the highlights and another over the shadows of your photo. It's the classic forerunner of the modern color grading panel and still a quick way to add mood. Warm highlights with cool shadows is the best-known combination.
Pick a tint for the highlights, a tint for the shadows, and use the balance slider to set where the transition falls. Split toning works beautifully on black-and-white photos: a warm sepia or a cool blue cast instantly adds character. In newer Lightroom versions this folded into color grading, but the idea is the same. Keep the saturation low, because subtle almost always works better.
Read also the in-depth explanation: Split toning 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.