You figured it out correctly - these brushes are used to define the qualitative ranges on the graph. You can review our online demo
where it is demonstrated how to set colors for them. Additionally you may follow our help topic
where the Colors are set for each element of the QualitativeRanges collection and the BulletGraph is created programmatically for simplicity.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>