That was the correct file for me and it actually worked! Had to update the path to A-10C_2 though, since I am using the new version they released.
Also found that you can change the HMCS Scorpion Helmet Display color by editing the materials["HUD_IND_GREEN"] in DCSWorld\Mods\aircraft\A-10C_2\Cockpit\Scripts\materials.lua
You can see in other parts of that file how it references that HUD color variable for the HMCS
Another note: you can test your changes by launching an instant action (Takeoff) mission. If it doesn't work, exit the mission but stay in DCS. Change the lua file again and then "Fly Again" - it should use your latest changes.