LeFusil is straight on about the potassium nitrate heat coloring. When you visit gunsmith shops in the UK you will see small 5 inch or so in diameter pans there with the cooled potassium nitrate that the use for heat bluing. I use a torch to do it as I have become adept at doing so, which is an art itself. Proper polishing is a must for successful outcome.
You will have to experiment in how and what you use to hold the screws that you bring into the torch flame (a very small flame) and that the time-ing of leaving the screw in the flame and allowing the heat color gun up to the head of the screw is where the "art" comes in. I normally lay the screw down quickly onto the flat of my vice to cool down. Being able to get the purple violet color of the end of screws that extend through the sidelock plates take a bit of practice. You should note that these small side lock screws (bridle screws and so forth) get to color very quickly and secondly if you want to preserve that special violet/purple color let the screws cool for several hours before you put any light oil on them as oil will darken them.
Sometimes I wrap the screws a couple of turns with small steel wire about 6 inches long to hold them in the flame and with larger screws I hold them with a light pair of hemostats.
If you are using potassium nitrate to temper springs buy one of the Harbor Freight infra-red "point at" temperature gauges. They are very accurate.
Peanut oil has a high flash point and sometimes I use it in spring making.
Wear eye protection--- full face cover when using potassium nitrate.
