I am trying to use QGIS to analyse the colour component of a photo. I have imported the photo as a raster layer, created a polygon of the area in question and have tried to use zonal statistics but this doesn't appear to work. When I use the zonal statistics function, I fill in all the relevant boxes and click ok. Then the message window closes. That's it. When I have tried using the function on maps I already have, after clicking ok the progress bar came up and after it finished I could find the resulting stats. When I tried it on my photo, absolutely nothing happens. The window just closes, there are no stats anywhere.

Ideally, the stats I would like to produce are the number of pixels in the polygon, and the number of pixels of each colour. Or, to assign the colours into similar groups say 0-25, 25-50, 50-75 etc up to 255. Of course, this is made more challenging by the fact that a photo used as a raster layer is made up of 3 colour bands and, as yet, I haven't managed to separate the bands.