Dull grey is an obviously cold joint so that's what I'm going to try for from now on!
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Dull grey is a cold solder joint if the solder has resin in it. Almost all solder's do have resin. Properly soldered joints force the resin to the outside of the joint and they wind up coating the joint and creating a insulation on the outside--a protective coating to reduce corrosion. A solder that is dull grey and is properly done, must not have this resin. I'm not familiar with any solder like this, but to some degree it makes sense. Resin is an insulator, so a joint with no resin could be better than a joint with resin. The only downside might be the long term effects of a solder joint without a protective coating. It would also be interesting to see how one would use such a solder, as without resin the flow might be difficult to handle (I don't really know since I've never even seen solder like this--and don't even know if it exists--just guessing by your description).
This is mostly speculation on my part--but it seems plausible.
Sinbad, I suggest you try samples of TRT Wonder solder and the WBT solder. Both of these are wonderful, and make a stunning difference in the sound when used instead of the common variety. If you cannot find these two, I can send you a small sample of the TRT for sure, and perhaps the WBT. I will have to look for the WBT, I am not sure I still have any around. Let me know if you want to do this test for yourself.