Check Options bar and set for desired sample size.
Click on the area of the image you want to sample and retain. Notice that the Foreground Swatch at the bottom of the Toolbox reflects your color sample.
In your Swatches Palette, click on the "New" icon at the bottom. Your newly sampled Foreground color will be added to the list of swatches.
Wanna manage sets of swatches? Explore EditPreset Manager…