And yes, that's a big reason why they went with stainless. Stainless steel does corrode, just much less visibly than regular steel. It also has different physical properties like work hardening etc. It's also VERY expensive. Salty napkin math says $10,000 more for the Cybertruck.
I was thinking about "molded-in color" panels like the old Pontiac Fiero. That makes sense, though, re: painted plastic panels. They'd be a whole different ballgame and there's, I'm sure, interaction between the paint chemistry and the plastic chemistry that's probably pretty hairy.
I bought 2 pairs of the same shorts a couple years ago, one in blue and one in olive. Both were used almost interchangeably in terms of exposure to sun, dirt, washing, and salt water. But on the green ones a section of the waistband is clearly differently colored where on the blue the colors still match.
This gives me a deeper appreciation for cars with plastic body panels. I also wonder if DeLorean Motor Company took any of this into account when they designed the DMC-12 with stainless steel body panels.