If the steel starts changing colors (temper colors) it's way hotter than you want for soft soldering.
That is incorrect.
You already said you're not a gunsmith, but do you do any metal work and heat treating? I do, and am not guessing.
Yes, I do flame heat treat steel from time to time. Lessee, Noticeable color change happens around 440F depending on light and eyes.More commonly you're looking for around spring temper, blue, around 850F.
Now eutectic tin/lead solder with a bit of silver added (helps the flow) melts at 370F. My favorite alloy for electronics and low strength applications. Heat the joint up to 440F and you'll burn the usual flux Brownell's Hi Force 44 flows at 475F so you have more latitude before your flux burns. But there is absolutely no point in heating past the flow temperature.
