From the Appearance panel, you can add a number of separate strokes and fills to any selected object. These can be gradients or solid colors. This combined with the ability to alter the transparency and blend mode for each stroke and fill applied, can be used to produce a range of different color interactions between upper and lower strokes and fills.
Strokes and fills listed in the Appearance panel are arranged by z-order (like layers in a layer stack) and can be dragged and dropped to reposition them within the panel.
To add additional strokes or fills:
With an object selected, click on the Appearance panel.
From the panel, select Add Stroke or Add Fill.
To edit a stroke or fill:
Select the stroke or fill in the Appearance panel.
Click on the stroke/fill color selector to alter the color or apply a gradient.
For strokes, click on the stroke width value to alter the stroke properties.
Click on the current blend mode to swap it for another using the pop-up menu.
To make a stroke or fill active for editing:
With an object selected, click on a stroke or fill from within the Appearance panel.
To reorder strokes and fills:
Drag and drop strokes and fills within the Appearance panel.