created a (box) chart, but want different colours.

Jason,

In a column chart there is really no reason to have the data points of the same data series in a different color. That kind of chart compares data of the same qualities, so all columns will be one series and, as such, one color.

If you want different colors for your bars, change the row/column assignment (Switch Row/Column). Then you will have different series and the legend will show different items, too.

Alternatively, click the data series and open the format dialog. Select "Vary colors by point", but this will still show only one legend entry.

Please update your profile to indicate your Excel version. Charting dialogs have changed significantly in Excel after v. 2003. The commands mentioned above apply to Excel 2007 and later.