Cold blue will only ruin what you have. DON'T DO IT.
I strongly question the business about the purple color caused by the metal, in this case.
I'll be blunt: Most of the time when I hear these stories about odd bluing colors, it's from a part-time bluer trying to explain away a bad job.
People who only run a bluing operation once in a while simply don't develop the skills to the level needed, and you often see either over-polished steel with rounded off edges and waves in the flats, or you see odd colors.
What causes metal to take on a purple or reddish color is one of two things:
First, the steel "can" cause it, either due to the type of steel, or the hardening.
Gun makers are WELL aware of this and don't use those steels, and take extreme care during heat treating not to have problems with bluing later in the manufacturing process.
If it was the steel or the hardening, the rifle would have shown purple colors BEFORE it was re-blued.
Second, and BY FAR the most common is a bluing operation that's "off".
Usually the bluing tank chemical mix is off, or the temperature is not under control.
Temperature is the single biggest cause of purple and reddish colors in bluing.
As for the business about the chemicals not being what they used to be.....BS.
Modern hot salts bluing chemicals were developed before WWII, and they are the best ever right now, simply due to better manufacturing control.
I'd suggest you talk to either Marlin, or just as good, one of the top re-finishers about this.
I'm sure they can explain this better, and can do it better.
I HAVE a re-blued Marlin 39-A and have seen a good many more over the years.
I've never seen one that was turning purple that wasn't a defective blue job.
If it was the steel, or "bluing chemicals aren't what they used to be" ALL of them would be turning purple.
They aren't.
Here's some professional re-finishers to talk to:
http://www.fordsguns.com/
http://www.gunbluing.com/
http://www.rebelgunrefinishing.com/