How do I calculate the regionprops of a georeferenced sattelite image within the boundaries of a shapefiles?

I was wondering if anyone could tell me how to obtain certain properties of a georeferenced map within the confines of a polygon coming from a shapefile. What I would like to do is to e.g. calculate the average color/intensity on a sattelite image within a specific county. I have been trying to build a mask from the shape polygon and use "regionprops" but cannot figure out how this works with a projected map. I would greatly appreciate any help or advice!

Thanks a lot in advance.

Best, Marcus