QGIS change zones in a raster to grayscale by polygon overlay

[المشرف العام]

I've got a raster ortophoto and a polygon layer:



I need to alter the raster layer as follows:

• Everything that is inside the polygon (overlaying) should stay as it is right now (in color).
• The rest of the raster image (the zone that is not overlaying the polygon) should be changed to grayscale.

I can't imagine right now how to achieve this..



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