Junipers should be fine in Flagstaff at this time of year, at the most it should have some shade in the afternoon, or dappled shade during the day. Yellow needles MAY indicate the the tree is too wet. You should only water it when the soil is dry on the top, but when you stick your finger into the soil up to the first knuckle, it is just damp still. Then water it very well, making sure that the water runs out of the holes. You don't need to mist a juniper, that I am aware of. It isn't a tropical tree.
Is the soil sort of chunky, or is it like dirt?
Are the needles yellow, or are they more like a tan color?
Is it only the needles toward the inner parts of the tree, or all the way to the tips?
Joanie