You can apply a picture to either the front or the side:
Create a picture in a graphics program (e.g. MS Paint) that is just a
block of colour.
In Excel, select the chart series that you want to colour
Choose Format>Selected Data Series
On the Patterns tab, click the Fill Effects button
On the Picture tab, click the Select Picture button
Find the colour block picture that you created, and select it, then
click Insert
For 'Apply To', check Sides or check Front
Click OK, click OK