In the case of an already blued gun, there's nothing you can do to make it turn purple or brown that won't ruin the finish.
As above, purple parts are a function of the bluing process being slightly off, and are actually a factory defect, since the idea is to have a BLUE/BLACK color.
True, the steel used can have an effect, but this is a non-issue in Ruger's since they very closely control the steels used, and they don't use metals that give "off" colors.
Brown colors are a function of an OLD gun that's been handled, worn, and simply used for many years.
The blue finish oxidizes and wears to a brownish color.
You can REFINISH a gun with the old black powder type browned finish, but this is not as easily done on a revolver as it is on a straight rifle barrel.
Uberti and Cimarron are now offering guns artificially "weathered" to look like old, well-used guns but this is more the look of a gun which has a thin finish, exposing silver metal under it.
Refinish, yes, convert the color, NO.
I know of no way to turn blued steel purple or brown without ruining the finish, and making an ugly mess of it.
Refinish to another color...yes. ALTER the color to another color....NO.