The colorimeter at your local paint store, looking at a paint chip from your wall, is very likely going to do a whole lot better than the process you just described. If you call the store, ask how big of a paint chip sample they need to match existing, weathered paint. It very well may be tiny (like, 1cm^2), and they should do this color match for free. You could buy just a pint, first, to see that it works.

I've been quite surprised by how non-uniform most digital camera imagers and in-body post processing color pipelines muck with the spectrum coming off the sensor, all in the name of producing a more pleasing JPEG. If you still want to do what you described, I'd use RAW and use dcraw to convert to tiff, and open that in gimp.