The coloring isn't intended to convey meaning, but simply to subdivide that dendrogram: the viewer doesn't need to recognise the colors or match them to a scal

The coloring isn't intended to convey meaning, but simply to subdivide that dendrogram: the viewer doesn't need to recognise the colors or match them to a scale, just see that they're different, and the colors used are distinguishable even when converted to black and white so they satisfy this requirement. This colored shading is arguably more attractive than simply using a range of grays.