please anyone could tell me if there is any way to start a 737pmdg flight with the cold and dark cockpit mode.
You can use this program for that.
For PMDG it's a little different. It'll sometimes start you up strangely even with the patch. Open up the menu when you're in the simulator and bring up the PMDG drop down menu, under there there should be a preferences choice, maybe "styles" as I remember. In one of the sub-categories you'll find a "cold and dark" on startup option. The switches are still not in true cold/dark setting all the time, but good enough!