Here's a frame screw from a revolver that I re-blued with Perma Blue paste. I sanded down the whole head and slot to shinny bare metal and cleaned it with 99% IPA before cold bluing. Think I applied Perma Blue 4 or 5 times and left it on for ~5 minutes each time before removing for the next application. It came out pretty dark.
Not referencing you specifically but you did post an example pic people can see but on cold bluing in general.
A lot of the "cold blue" performance has to do with the steel composition and hardness ( that's where some of the red and purple comes from). It isn't always easy to determine this and sparking is more holistic than an actual cook book in terms of reactance. Some metals simply wont take it well at all.
Also, these formulations are barely 5% of the hot tank counterpart ( but they also don't carry all the danger or damage potential either)
clean it, heat it and layer it for overall best performance