The solution for me was:

1. I used the Feature to Point-tool on my polygons and centered the points 'inside'.
2. I exported the polygon shapefiles to bitmap with worldfile attached (low resolution and 24-bit true colour) to get the RGB values as raster values.
3. I loaded the bitmap and the point file and used the Extract Multi Values to Points-tool to get the values in the point attribute table.

The solution for me was:

1. I used the Feature to Point-tool on my polygons and centered the points 'inside'.
2. I exported the polygon shapefiles to bitmap with worldfile attached (300dpi/24-bit true colour) to get the RGB values as raster values.
3. I loaded the bitmap and the point file and used the Extract Multi Values to Points-tool to get the values in the point attribute table.