You realize that makes it sound more likely that your workers (sorry, contractors, whatever) are poorly paid, right?
We pay whatever they ask. We don't really have the volume or size to impose anything on them.
Great, that means there is probably a fantastic opportunity to separate your brand with something like this and use it as marketing leverage against those folks ;-) Win-win?
And, I could see this as a way for painters make a name for themselves, if they could build up a portfolio that demonstrates their particular talents.