In a previous part of my career we were sometimes required to climb as high as the aircraft could hover without breaking any engine limits and remain there for some time (usually about 10,000 feet or so). One night we were required to be a little higher, about 14,000 feet. I suddenly noticed that the orange sodium streetlights below us no longer looked quite so orange, and my vision became mainly monochrome, like black and white TV. I realised it was likely to be an early symptom of hypoxia. Thankfully, we were able to descend shortly afterwards and as soon as we had gone down a couple of thousand feet, my colour vision returned to normal.
If my memory hasn't completely failed me (and it might have), I think at 14,000 feet the air (and oxygen) density is only about 55% of that at sea level.