How do they prove "willingness"? Like if you put $100K-$700K, but you just don't have any employees that are good enough for $700K, how would you prove you're nevertheless willing to hire someone (the right person) at that salary?

Also, what about the converse - what if the range is actually wider? Does anything prevent them from putting something narrower?

True, I did not notice that CO required a "good faith" range for the job. Ideally, the law would be that the employer simply has to state a range and then let the market forces dictate the ranges employers advertise. In competitive labor markets, the bottom of the range will naturally rise, and if they do not, then sellers of labor know that it is not a business worth going into.