The Humbrol is a better match than the Polyscale in my opinion. Tamiya is pretty good as well, although I prefer the Humbrol. Only problem I have is finding it in the States. I find that the Polyscale "Concrete" is a bit too blue-gray and the "Aged Concrete" is a bit too yellow. I get a better approximation by buying both and mixing them together.
One little secret I saw a long time ago was to use a salt wash on the surface before the paint completely dried. I saw an "old codger" do this when I was--well, not yet a codger myself...
He mixed regular table salt with water and washed it unevenly over the surface. It mixed a bit with the still tacky paint and created a great "mineral stain" look when it all dried. Adding the wash before the paint dried completely created a softer "edge" between the lighter stained areas. I tried this a few times myself with mixed results. It is something I always intended to go back to for experimenting.
Another wash that I use for mineral stains is finely powdered white pastel chalk in alcohol. I add very little chalk to a 35mm film can of denatured alcohol. As the alcohol dries, it creates uneven areas of lighter stain.
Tom F