You then 'paint' the color back in using a brush tool, restoring the original colors into the areas of the photo that you want to.

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Nasa's Mars rover Curiosity has used its brush tool for the first time to sweep dust off the surface of a rock.

BBC: Curiosity Mars rover deploys its rock brush

That latter tool, the Advanced Healing Brush, lets you paint in corrections, rather than using the round dropper to make tweaks.

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