Hi Christian,
Unfortunately, your requirement is not supported, even with CSS. Even if you defined different CSS classes for each colour, I'm not sure that you could dynamically assign them to individual chart columns via scripting, based on the a specific measure value.
Conditional formatting is based on the same measure that is plotted (Column A) rather than an additional measure (Column B), so this feature will not meet your requirement either.
The issue is discussed further in the following posts:
Conditional Formatting Over all measures inside Chart
Charting enhancements in design studio 1.5
The only possible options I see are:
1. Review the SDK Component List to see if any of the charting components meet your needs;
2. Develop your own SDK chart component that allows the desired colour formatting.
Regards,
Mustafa.