From Raster to Masked and Filtered Polygon Vector

Question

I have a set of rasters (geologic risk) from which I want to extract zones with a specific color, clipping them with a polygon (administrative borders), outputting a shapefile.
I'm using QGis (and GDAL from command line).
I know I can use clipper, polygonize and show only features with the value I need, but:

1. it takes ages to polygonize the raster, and I don't want much of the information in it;
2. clipper doesn't work because the polygon border is larger than each raster, but fits inside the sum of all rasters
3. to make things complicated, the zone I would like to extract don't have the same color value in all rasters (but do have the same rgb color)

How can I solve it in a fast way?

Answer 1

I think I got my answer: Filtering a raster by pixel value with GDAL

I simply have to rasterize my polygon (I addedd a field with value=1), then use raster calculator to do:

mask * map = color value

then I can use polygonyze (and join)