For what it's worth - I asked the founder of the large regional "light touch" car wash in the PNW about the possible discoloration and here's his response:
Thank you for your email. For the last 30 years our mission has been to develop safe Cleaning products for your car, our employees and the environment. The Kaady Chemical car wash shampoo used in the Kaady wash process to wash your car has a PH of 11. and is non-corrosive, free of caustics, acids and phosphates. We are continually searching for the latest technology to incorporate into our formulas. I'm sure you have noticed the brilliant shine of your car after washing. I would like to bring to your attention that most touchless car washes ( high pressure water ) use a two step chemical application. High PH is usually caustic based ( corrosive ) with a PH around 13. and a low PH acid around 1-2. in a lot of cases Hydrofluoric or Ammonium Bi Fluoride.
Living in the rainy, muddy, but beautiful PNW I've always been okay with the cons of using a "light touch" carwash. I've washed my new black Model Y PUP twice now with no discoloration or any issues at all.