Changing bar colours:
Select the chart.
On the table, select the small coloured square for the colour you wish to change (circled).
Click the Color Well indicated by the arrow above to open the Color palette.
Choose the new colour.
Repeat for each bar you want to change.
For a wider colour selection, use the Graphic Inspector and the Color pane. See the Numbers '09 User Guide for details. The guide may be downloaded via the Help menu in Numbers '09.