Before adding color to any design, make sure all of the lines are connected and there aren't any gaps in your design. To fix any unconnected lines, you can use the Designer.

If the shape's lines are all connected properly, from the Customize & Resize step, first click the Customize button, then Color.

In Color you are able to choose the material by clicking on the Change Swatch button in the top left and scrolling through glass, fabric and wood swatches. To add a swatch to the design, simply left click on the area you would like filled with that swatch. If clicking on it doesn't fill the space, make sure that it's a closed area you are trying to fill (you can fix any not connected linework in the Designer).