Retouching Tools: Eyedropper Tool

If you are retouching a certain area with the Paintbrush Tool, you can use Photoshop's 'Eyedropper Tool' (I) to sample a color from the surrounding area.

If you are retouching a large gradated area you will want to set your brush value to a lower opacity and resample in the lighter and darker areas of the tonal gradation.