Product: TIBCO Spotfire®
How does Spotfire determine what font color to use?
When creating a visualization such as a KPI chart, Spotfire must determine whether the text should be white or black to produce the best visual contrast.
In general dark colors will have white text, while lighter colors will have black text. This link provides more information about the algorithm used.
For users who want more control over visualizations there is a popular suggestion in the Ideas Portal.