Hi, I am still playing around with this as I am not yet satisfied with the results.

I had to find a workaround for editing the main texture's alpha channel as normally as this causes display errors with the fuselage, especially in the window area.
Tried editing the Spec-Files and their alpha channels and the bump files and their alpha channels. A combination of this, in addition to the right aircraft belly color (for the chrome effect) is finally the solution.

I was never satisfied to "paint the chrome effect" on the aircraft body, is this is too static and colors are not shifting while flying a curve etc.