Ever spent ages tweaking a color to get it just right - only to need it later, after you've already lost it in a gradient or mask, and the eyedropper can't capture just the right color? Well, even if the situation isn't that dire or complicated, it's still convenient to have frequently-used custom colors readily available. To that end, you can save any color you want to the color Swatches panel.
Once you've used the color slider or eyedropper to select the color you want to save, just click the menu tab in the upper right-hand corner of the Color/Swatches panel (the little tab with the three horizontal lines and the downward-pointing arrow). Click Add Swatch. If you open to the color picker or click to the Swatches tab, you'll see your new color now listed among the default swatches.