After defining chart colors for a field on a table and stacking the columns, the colors are not in the expected order. What determines the order that the colors are displayed in?
The colors are ordered by the Chart Color "Value" field. See the chart above. In this case the columns stacked by incident.priority, and the color associated with each priority is defined in sys_report_chart_color. The colors will appear, left to right, based on the value.
Redefine colors that the system uses in reports under Reports > Administration > Color Definition.