Tessa @Asset218, what do you hope to gain by getting even more opinions on this, 5 months after the fact?
If you were unhappy with the work, that should have been handled shortly after the work was done. It sounds as if you are now trying to build a case against this contractor by collecting as many "professional opinions" as possible.
If I were in your contractor's shoes, I would be very put out if a customer came back to me 5 months after I washed their house and told me to 'fix the siding I screwed up'. I stand by my work and would be more than willing to make a situation right if I was told shortly after completing a job that they were unhappy with the work. But 5 months? A lot can happen in 5 months.
My personal opinion doesn't really matter, and as someone fairly new to this field, it carries much less weight than the pros who have already replied to you.
Personally, I think that the oxidation issue is something that cleaning contractors should understand well and communicate to their customers. I say "should", not in any legal sense, but in common business sense, for the good of their own businesses and the satisfaction of their customers.
My mechanic should know that my new car takes 0w20 oil- but if I go to the old-timer down the street who's been doing oil changes for 60 years, I'm not going to assume he'll know that automatically. That weight oil is a rather new and unusual choice for a car engine. Just because someone has been doing something professionally a long time doesn't mean they've kept themselves up to date in their education and research.
The fact is, many power washers don't understand oxidation, or perhaps don't even see the resulting discoloration as being a problem.
The bottom line is, caveat emptor- buyer beware. In most situations like this, where no damage has actually been caused, it falls on the customer to ensure they're getting what they really want out of a business exchange. Do the research before hand, make sure you hire a company with good reviews and references, that stays up to date in educating themselves.
And while it's true that there's danger in paying too little for something, a high price does not automatically mean quality. You have to dig deeper than that.