HighlightLevel

is an option for DendrogramPlot that specifies the level at which to highlight the dendrogram.

Details and Options

To use HighlightLevel, you first need to load the Hierarchical Clustering Package using Needs["HierarchicalClustering`"].
Valid settings for HighlightLevel are None or a positive integer less than or equal to the number of clusters.
With the setting HighLightLevel->n, n distinct clusters are highlighted.

Dendrogram highlighting the top 2 clusters:

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