Hi John L,
I bought a little butane torch at Rio Grande, but they also have them at Home Depot and small less convenient ones at Radio Shack.
Heat the screw head slowly as possible and keep a cold quench close...I use the mug of ice water I have on my workbench or martini, both work.
As soon as the screw starts to turn goldish, take it out of the heat...watch it carefully as it will continue to darken....quench immediately to stop the coloring process as you approach what you want.
When heating, hold it in the far front of the softer colored flame,several inches from the torch,, and keep the torch flame as small as possible and still keep it burning,,, take it out and check it, back in the flame, out to check it.....it takes some practice to get used to when to take it out of the flame, and when to quench but you will develope a feel thru practice.
Also, you can buff the screw to remove most of the color if you want to reuse a non acceptable screw.
It isn't much metal and even butane is pretty hot....Mapp and oxy-acetylene are too hot and a full size propane torch is pretty hot. The smallest flame on a jeweler's size butane is still mighty hot but the least hot of the choices...
Good luck!