Tips about the Semi-Automatic Classification Plugin for QGIS
Activating the Classification preview pointer you can display a preview with a left click on the image. Also, with a right click you can display the Algorithm raster that represents the minimum spectral distance (calculated by the algorithm) of a pixel signature to any training spectral signature. Dark pixels are more distant to any spectral signature and possibly a new ROI should be collected for these pixels.
For any comment or question, join the Facebook group about the Semi-Automatic Classification Plugin.