Case color is the by product of the case hardening process. It is used to provide a wear resistant surface to an otherwise soft steel by adding carbon.
Usually small parts are not case colored. The temps involved could cause warping.
The band on the pen does not have to be hardened. So any method of coloring the steel would be acceptable. Some liquid cold blue and a propane torch should do just fine. Just a propane torch. Some liquid plumb brown and torch. There are many variations on this. Brownells sells some Birchwood Casey touch up pens, black, green, red.
They also sell the salts for a nitre blue, which only requires heating the salt to about 550 degrees. So you could blue the barrels yourself
Pete
