The reason you saw white is that white is the default background color.
And without setting an actual pattern, you can't change the background.
Double click your series, click on the Patterns tab, click the Fill
Effects button under the fill color choices, then click the Patterns
tab. Select Foreground and Background colors, then pick the 90% pattern
(bottom of second column). You see the occasional dot of the background
color on the sea of the foreground color, but it's the best you're going
to get. Click okay until you come back to the panel where you can check
the Invert if Negative box, and check it. The fill will now invert
between 90/10 and 10/90.
Andy has described the technique behind my Conditional Chart example:
http://www.geocities.com/jonpeltier/Excel/Charts/format.html#CondChart
- Jon