Well don’t think that it is a general problem with payware planes.
At the mooney, carenado decided to integrate the livery directly into the model, instead of outsourcing the livery separately, as almost everyone else does.
So you had to load the complete model, in an 3d Programm so to speak, then change the livery directly on the model itself and save it as a new modell.
With the mooney i managed to change colors, but the pattern where it occurs and so on stays the same because it is indicated directly in the model where which color is displayed.
So without changing the 3d model you can do nothing else except change one color from e.g. blue to white.
On the picture below you can see where I could change colors.
There are 4 places in total that can be colored.
i used extra extreme colors to show where which pattern ends.
Each color in the picture is an area that you can color as you like.
The 4 colors are then also reflected on the wing tips and the fin.
The gray of the aircraft is embedded in the model and cannot be replaced without changing things directly in the model itself.
Its just extreme extra work and not as easy to handle as e.g. just change a few textures that are reproduced on the model, as almost all other manufacturers do.
So it is not a general MSFS2020 problem but a decision by carenado to do so.
Other payware manufacturers can of course go other ways and make things easier.!