These clouds need the very frigid regions of the lower stratosphere some 15 – 25 km high and well above tropospheric clouds. They are so bright after sunset and before dawn because at those heights they are still sunlit.
From a dark site, you shouldn’t be able to see clouds at night. Just some black patches obscuring the stars. If you look at the clouds from the city, you will see them coloured in orange or yellow. But, there are some clouds that appear white or pale electric blue even from a dark place. Those are noctilucent clouds or polar mesospheric clouds. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator. They can only be observed when the Sun is below the horizon. I’ve seen them twice from Romania. Date and place: June 21 2009; Omu Peak, Romania Camera: Canon EOS 5D Lens: Canon EF 70-200 f/2.8L Exposure: 2.5 s Aperture: 4.5 ISO: 400