NLCs shine because at their high altitude they are still in daylight.
Stratospheric ozone absorbs some of their scattered light to confer a blueish colour
Noctilucent clouds are 80-85 km (50 – 53 mile) high.
Small, 0.1 micron, ice crystals compose them. Very low temperatures below -123 Celsius are needed to form ice in the dry mesosphere.
Nuclei on which the ice crystals can grow are also needed. Meteor dust, lofted oceanic salt, volcanic ash and rocket exhausts are all candidates.
NLCs are possibly becoming more frequent. Paradoxically, as the lower atmosphere warms the mesosphere cools.