My urine was very light yellow the whole time I was fasted, even though my mouth so dry at the end that I was barely able to open it..:) Yes dark urine is a good indicator of dehydration but not the only one. There are several to be considered.
As for dehydrated cells, that is part of the reason dry fast is so powerful in healing. Dr. Filonov explains several times on his site that cells turn into little ovens and burn toxins internally. That is one of the main difference between water and dry fast.