What you do wouldn't fade the paintwork itself (at least I don't think so).
What I mean is, after one clean you'll take away any polish and the polymers in it.
This means the car has no sun protection, and will fade over time.
How long depends on how new and how good the paint is.
But a Bmw will still fade like a Lada, honest!
You've surely seen ex-red cars going about which are a flat pink.
This is what happens to cars which aren't polished.
I've had it happen in as little as two weeks, depends how sunny it is.
It mainly affects top surfaces, ie:bonnet, boot, roof.
Shouldn't be a problem on modern metallics, but I don't know how pure water will affect the laquer.