For a long time, whenever I built a color palette in Illustrator, I'd create Swatches for each of the colors by selecting them one at a time and clicking the New Swatch icon. At first, I didn't think much of having to add colors one at a time—I just thought that's the way it was done—but after doing this so many times, I began to wonder if there was in fact a better way.
It was then I discovered that it's possible to create Swatches for multiple colors at the same time.
How it's done
There's actually two ways you can do this:
Select the objects you want to grab the colors from, go to the Swatches Panel (Window > Swatches), open the Swatches panel menu and click Add Selected Colors.
Make sure nothing is selected, open the Swatches panel menu and click Add Used Colors. This will automatically create Swatches for all of the colors you've used in your document.
What's great about both of these methods is that the Swatches are created as global colors, which means they'll automatically be updated throughout your artwork when you edit them.