I've seen a few 20,000 ft peaks with dark yellow urine stains.
I have not been to the top of Everest, but a little known fact is that Edmund Hillary peed on the summit. I imagine his was dark. Physiologist Griffith Pugh (at basecamp) tried to teach them that hydration was important, especially up there when just breathing is dehydrating: lung air pumped out like a race horse at 98F and 100% humidity, and replacing it with inhaled cold, thin -20F and 5% dry air.
I mention this because it is not a yes or no situation...while dehydration is to be avoided, some degree of it can sometimes be tolerated for a while even with extreme situations. On the flip side, hydration alone will not prevent AMS. But it does reduce the risk. Risk management.
Salty mentions getting into the groove at about 5 days. Sounds about right.
