I used an illustration software called Affinity Designer
to create my paint scheme. There is definitely a learning curve, but with practice you can get good results that your painter can use. In fact, he used the vector files created by Affinity Designer to make vinyl masks for the tail logo and the the checkerboards.
Here is the picture that I created and gave to the painter:
Here is a more detailed version that I have used for other things:
And here is the final product: