One of the most common questions I hear from designers when I'm teaching Adobe Illustrator is, "What's the "Global" checkbox for in Swatch Options?" (Swatch Options is the dialog box that opens when you double-click a color in the Swatches palette so you can modify the color mix, or when you choose New Swatch from the Swatches palette menu.)
When you tick the Global checkbox in Swatch Options, you turn that swatch into a "linked" or "smart" swatch. From then on, filling or stroking an object with that swatch links the object's color to the swatch's definition, equivalent to applying a paragraph or character style to text. Modify the swatch's color mix via Swatch Options and you'll see all the swatch-colored objects in your illustration update their color to reflect that change.
In other words, global swatches act just like ones in InDesign's Swatches palette or QuarkXPress's Colors palette.
Non-global swatches, on the other hand, are like InDesign's "unnamed" colors (the ones you can mix from ID's Color palette), except that in Illustrator, they exist side-by-side with global colors in the same Swatches palette. Adding them to Swatches doesn't automatically make them global, as it does in InDesign.
As with ID's unnamed swatches, you can color objects using the non-global swatches, but they're not linked to the swatch. Modifying the swatch's color mix doesn't do anything to existing objects using that swatch, they remain at the "old" color.
You can tell at a glance in Illustrator's Swatches palette which ones are global and which are not. In the palette's default Small Thumbnails view, global swatches carry a little white triangle flag at the lower-right corner of the swatch thumbnail, non-global ones do not. When viewed as a List, global swatches have a little gray square to the left of the icon denoting color mode (CMYK, RGB etc.), non-global ones only have the color mode icon.
So now that you understand how to make solid color swatches global, and why you'd want to, what about gradients? If you double-click a gradient swatch to turn on the Global checkbox, you'll see that it's impossible — except for the swatch name, all the fields are grayed out. The Global checkbox is unchecked.
But don't let that throw you — gradient swatches *are* global in Illustrator by default. Since the color stops used in a gradient could be a mix of global and non-global colors, it's probably just too hairy for Adobe interface designers to figure out a way to convey exactly what's going on via little icons.
To prove to yourself that gradients are global, and to make global changes to a gradient swatch so gradient-filled elements in your illustration automatically update to match a tweaked gradient, use the Replace Swatch feature.
1. Fill a few objects with the same gradient swatch from the Swatches palette (either one of the default gradients or one you've created and added yourself). Leave an object selected to see a live update of the edits you're about to do, or deselect everything, it makes no difference.
2. In the Swatches palette, make sure the gradient swatch you used above is selected.
3. Open the Gradient palette to edit the swatch. If you don't see a large square thumbnail of the gradient at the upper left of the palette, choose Show Options from the Gradient palette menu so that you see it.
4. Below the thumbnail and other settings, the Gradient palette shows a horizontal preview bar and the color stops used by the gradient. Edit the gradient here in the usual way, by modifying the colors for one or more stops, or by adding or removing stops, etc.
Notice that the Gradient palette thumbnail (the large square) is constantly updating to reflect your edits, but the one in the Swatches palette is not, nor are any unselected objects to which you had applied the gradient earlier.
5. When you're done editing, replace the old gradient swatch with the modified one. To do this, hold down the Option/Alt key and drag the large, square gradient thumbnail out of the Gradient palette, all the way to the Swatches palette until it's hovering over the original gradient swatch (from Step 1). When you see the original swatch highlight under the outline of the one you're dragging, release the mouse button, then the Option/Alt key.
The gradient swatch in the Swatches palette is replaced by the one you dragged and dropped on top of it, and all the elements in your illustration that used that gradient reflect the new gradient's settings.
The Replace Swatch method also works for global pattern modifications:
1. Fill a few objects with a pattern from the Swatches palette, one of the default ones or one you've created and added yourself.
2. Drag and drop the pattern swatch from the Swatches palette out onto the artboard, where it magically transforms into a single, editable pattern tile.
3. Edit the tile's fills and strokes with the Direct Selection tool.
4. To replace the old pattern swatch with the edited one (and update all objects in the illustration using it), hold down the Option/Alt key and use the Selection tool to drag and drop the tile on top of the old pattern swatch in the Swatches palette. Voila!
Making Non-Global Elements Global
Unfortunately, the Replace Swatch method doesn't work with objects that are already filled with a non-global process color (spot colors are global by default). Replacing a non-global color with a global one doesn't link previously-filled objects to the new color, nor does simply turning on the Global checkbox for the non-global swatch (the first thing everyone tries).
To link these non-global-colored objects to a global version of the same color, you'll have to do it manually. First, turn that color global by turning on the Global checkbox in Swatch Options. Then select your objects and click on the global color to fill or stroke them with it.
If you have a lot of objects you want to "turn global," select one example object and use the Select > Same menu to have Illustrator root out and select every object in the illustration with the same fill, stroke, etc. as the selection. Then, click on a global color swatch for each found set.