I got it to work. I think the instructions on the QGIS 2.6 for macOS installer's read me are not 100% correct. There we can read:
For the Value enter:
but it turned out I had to enter 2 PATH variables and not only one. Plus, the GDAL version ...
It appears more of a limiting factor due to RGB color band assignment and color matching due to several factors.
Hexagon Geospatial provides an article Supervised Classification - Signature Editor - color chips where they provide a diagnosis to a similar issue you are facing.
Sometimes the Signature Editor - color chips do not match image ...
That is noise / offset pixels or unexpected dark or light strips fringing an object.
Data from airborne multispectral or hyperspectral imaging scanners shows a pronounced striping pattern due to varying offsets in the multielement detectors. This effect can be further exacerbated by unfavorable sun angle. These artifacts can be minimized by correcting each ...