Invert If Negative

Both column and bar charts give you the option to invert the area colour of any bar that has a negative value
Start by double clicking the series, in order to display the Format Data Series dialog, and on the Patterns tab check the Invert if negative option.
SS
Click the Fill Effects button and go to the Pattern tab.

Use the foreground as the negative colour
Use the background as the positive colour

There is no way to get solid colours they have to be a 5% pattern