Next to the persistent color option in the colors tab, I would prefer to load rgb values associated to your dimension in the load script. Load three fields e.g. Red, Green, Blue and in the background color of the dimension (click on the plus sign before the dimension) use the expression
1. In order to ensure consistent placement of colors, use the "Persistent colors" check box in the Colors tab for all related charts. In this case, colors 1-18 will get assigned based on the Load Order of your dimension. Arrange the values in the desired order and assign chart colors accordingly.
2. Use "Background Color" attribute for the first chart expression and assign color there, based on the value of the dimension. You can either use nested IF conditions, or associate the RGB formula with the dimensional value and use it - something along the lines of :
where ColorFormula is a field associated with the dimension (region) and contains the RGB definition of the desired color.