and some additional information from the original post......
So as not to make you guys wait 5 years for me to build something with the trigger guard and then wait for it to tarnish, I sped up the process with a couple of brass darkening agents to see how the solder joint would fare.
The guard itself was colored with the "Historic House Parts Brass Darkening Solution" Jim uses for both brass and steel. As the color developed, you could see the joint very well, but with a little more soak time the solder also colored dark. So, in a natural tarnishing situation, I would expect the joint to show for a while and then, possibly, fade.
As another check, I tried three different solutions on the solder alloy itself, the "Historic House Parts" solution, Birchwood Casey "Brass Black", and Birchwood Casey "Super Blue". As you can see in the photo, all seemed to color the solder itself very well. The BC "Brass Black worked the quickest and darkest.
I'm not sure what solder flux I am using. The label fell off the jar about 25 years ago...
It is, however, just a common water soluble, borax based flux.