If there was an easy way to do that in FSX, we would do that.
We USED to have a Paintkit for FSX, but nobody really understood how to use it, because it required to change multiple .CFG files for each vehicle and to understand the scoring rules.
And, if we released an update to the default .CFG files for the vehicles, it would require all custom vehicles to be manually changed as well with the new additions. Not adding these changes would usually results in errors, which resulted in increased support because users forgot to add something to their repatins that might have been required by an update of the code.
Also, in FSX, adding a single airline would result in hundreds of new Simobjects variations added to the simulator, resulting in a constant slow down of the startup time, the more and more liveries you added.
That's why we discontinued the old PaintKit and created a completely new customization method in P3D V4 that doesn't have any of these issues.