This feature is highly desirable for us. For Category datasets, I would suggest two ways of assigning a chart series color from a field:
1. Specify a field whose value is the actual color to be used when plotting the chart category, e.g.:
"My series"
$F{myCategory}
$F{myValue}
$F{myCategoryColor}
... $F{myCategoryColor} would contain an HTML color string (e.g. "#FF0000") to use as the category color.
2. Specify a field whose value is an index to pick a specific color bucket from the series color, e.g.:
"My series"
$F{myCategory}
$F{myValue}
$F{myCategoryColorIndex}
...
... $F{myCategoryColorIndex} would contain an index to pick a color from the series color. e.g. if it has the value "1", the color of the bar would be green. This way, we would still be able to use chart themes to specify the colors, while ensuring that the colors used for each category are consistent from one chart to another regardless of the order and number of items.
I am not sure if this feature would make sense for other types of chart datasets, besides Category datasets.