Why does a cold solder joint appear grainy or dull?
Sometimes you can see a difference in the appearance of the metal of a bad solder joint.  I think this is due to something having gone wrong as the solder was solidifying, like one part of the joint was moved with respsect to another.

It seems to me though that the majority of bad solder joints don't look different on the surface of the solder.  If there is a visual clue, it's usually that the solder didn't wick into the joint properly.  It will look a little beaded up instead of having formed a nice miniscus with the edges having flowed along the metal of the joint.

Another point is that good solder joints don't actually look nice and shiny.  The solder will look shiny when molten, but develops a slightly dull finish upon cooling.

All in all, I think what you are referring to is more superstition than fact.