What utility do I need to create a skin for a plane?
There should be some pointers here for you ➡
But does this apply to FSX?
Aircraft pain is done in photoshop or maybe gimpshop in dds format (i beleave thats dx texture design) I beleave you would simply make a new folder in C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes called texture.5 or whatever number its up too. I would suggest using the existing textures to create an entirely new one or replace an old.
Available textures are defined in the aircraft.cfg i beleave and named accordingly. Pretty easy stuff right there.