Color is an important means to display categorical data in statistical
graphics. Categories are often hierarchically structured in a classification
tree, but most qualitative color palettes do not take this hierarchy into
account. We present a method to map tree structures to colors from the
Hue-Chroma-Luminance (HCL) color model. The HCL color space is known for its
well balanced perceptual properties. Our study suggest that hierarchical
qualitative color palettes are very useful: not only for improving standard
hierarchical visualizations such as trees and treemaps, but also for showing
tree structure in non-hierarchical visualizations.