Its not that difficult, the app I mentioned gives you percentage starting points to mix for the shade you need. To get your color ratio you only need to mix a small amount until you have the right mixture for color you desire. Save that ratio to mix larger quantities. Once done I mix three parts paint to one part acetone. I add a product top (the top of the can) of Japan Dry and one top of oil base hardener catalyst activator to give more shine and harden the paint to a stronger finish. I use a cheap HF purple air gun with an inline moisture separator and a pressure regulator. 35psi has worked well for me but this will depend on the viscosity of your final paint mix. You can look up how to mix paint to prep on Youtube.
The mixing app on Google Play is called color mix by Idea Matters. It has a camera mode included to capture your color
Don't hesitate to ask if you need anything, I was leery of it too but its not that difficult. Shoot some sheet metal as a test piece for color match and to make sure you don't get orange peel.
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