Click the fill color or stroke color in the Properties panel and, with the Swatches option selected in the panel that appears, click the New Swatch button at the bottom of the panel that appears.

Double-click the new color swatch in the list of colors, change the name (if necessary), and set options in the Swatch Options dialog box. Click OK to save the color swatch.

Apply the new swatch by selecting artwork and clicking the fill color or stroke color in the Properties panel and selecting the swatch.