Open your vector doll in Illustrator. I added some outfits in flat color to my doll. We're going to add gradients to liven up the color, work with the Appearance panel and apply patterns over the gradients, and talk a little about creating graphic styles.
Part of the reason why I like to block in colors ahead of time is so that they are easier to select later. Let's say I want all of the areas blocked in purple to change. I click on one of them with either the black or white arrow tool (Select or Direct Select), go to the Select menu on the menu bar, and choose Select Same Fill Color. You can use any attribute to select.
Once you've selected the areas you want to change, click on a gradient swatch. You can click on the gradient symbol on the left menu bar as well as the gradient palette on the right side of the screen to edit the gradient. If the gradient is linear, you can switch it to radial and vice versa. There's a white bar that shows up when you select the gradient tool. Use this to adjust distance, color, color spacing, etc.
I apply gradients to the whole doll, as seen in the image below. The gradients are subtle but you could certainly make them more pronounced. I have a radial gradient on the hair, face (including eyes & lips), body, and base, with a linear gradient on the outfit and shoes. Like anything else, play around with what looks & feels right.
Here's the doll with the gradients filled in. I think this looks better than the flat color, but why stop here?! Illustrator has some beautiful built-in patterns as well so I'll show you how to use those as well.
Next, we're going to give the jeans some texture. Illustrator has the ability to apply multiple fills to one shape. I'm not sure which versions of Illustrator have this. I'm using CS4 and I think CS3 (at least) has it as well. Earlier than that, I'm not sure. The Appearance palette is where we can apply multiple fills, and it looks like a small circle on the menu on the right of your Illustrator screen.
If you like this, save it as a graphic style. To do this, make sure your object is not in a group. Use the black arrow to select the jeans. Go to the Graphic Styles palette. It looks like a square with two smaller squares. With your object selected, click on the New button. And that's it! New style to work with.