Chrome moly will turn that dull gray color naturally. Scrub an area with Skotchbrite and see what happens. I don't know if that's definitive, but it will give an indication.
I just found my notes from that seminar. The high points:
Preferred TIG rod is ER80 SD2.
Also acceptable is 4130 VM. It must have the VM suffix for TIG use, and is 5 to 7 times as expensive as ER80 SD2.
The ER80 filler can be used to weld 4130 to 1018, or 1018 to 1018.
It's very important to have no moisture in the area of the weld. The arc can disassociate hydrogen from the water and cause hydrogen embrittlement in the joint. For that reason it's recommended that you preheat the area slightly - 150* or so - to remove any trace of moisture.
For more information on TIG welding 4130, go to www.tigdepot.com.
That's the site run by the seminar instructor.
I've never heard of MIG welding 4130, but I asked that question on the tigdepot site. I'll post the answer when I get it.
I do know that you can oxy/acetelyene weld 4130. As a matter of fact, there are old timers who will tell you that that is the ONLY way to weld it - TIG gives inferior results. You use plain 4130 filler. I don't know if the ER80 filler will work. That's what I would use if I didn't have the TIG machine.
I also posed the question of identifying the base metal on the tigdepot site. I'll post that answer too.