I have a shapefile layer of postal codes in The Netherlands. I also have a energy consumption colour map raster, with different energy types as different colours. I need to find a way of overlaying one on the other and extract the postal codes underneath the raster. So I want to extract each postal code that's coloured blue.
Welcome to GIS.SE! The tool you are looking for is called "Zonal statistics" and can be found under "Raster analysis" in the Processing Toolbox:
Select your shape layer with the parcels as Input layer and your Raster layer (likely one with classes not with bands like in my sample case):
as well as the statistics to calculate (I think Mean and Majority might make sense in this case):
The resulting vector layer has the statistics as attributes
that can be used for further analysis etc.
In this sample case (labeled with fid's) consider the hight majority values in the blue chanel, so you would have fid=5,9 (or postal codes rsp.):
Related: Overlay polygons on raster colour map to extract area values for each colour within each polygon
