I suspect dark urine may SOMETIMES mean you are drinking less water. I don't necesarily think it means you are dehydrated. I think being thirsty means you are dehydrated. There are no studies at all or scientific reasons to suggest people are commonly dehydrated even though they are not thirsty. So the old paranoia about dehydration is mostly just paranoia. If you are not sweating a lot and are not thirsty, you should be fine. There is no evidence to the contary and I personally trust my brain to be smart enough to tell me when my body needs water. After all, that is the very purpose of the sensation of thirst! How stupid would it be of nature to not make you thirsty in a timely fashion when your body needed water!!!
As for dark urine, any time you consume vitamin rich food like liver or vitamin pills, you can make it darker. It just means that the amount of vitamins and minerals in the urine are more concentrated compared to the amount of water. There are two ways for that to happen. Either you pass less water or you consume and thus excrete more vitamins and minerals. Eating paleo is all about eating vitamin and nutrient rich foods, so I would actually expect paleo eaters to have darker urine than nonpaleo eaters. No matter how good your digestion, you never manage to metabolize all the nutrients you consume. Some always escape into the urine.
So to summarize, yes, darker urine 'could' be due to dehydration, but if that were the case, you would know because you would also be feeling thirsty! Or darker urine could be caused by increased intake of vitamins and minerals which is likely on paleo if you are doing it right. Also, it could be caused by specific foods that darken urine, like asparagus, or it could be caused by various illnesses or medications.