It's all down to the temperature and humidity. The military regularly brief contrail altitude as they help the enemy see you. Occasionally you can see dark contrails above white ones if someone flies level at the same altitude as a thin layer of cloud. This is caused by the exhaust precipitating water vapour out of the cloud and forming ice crystals which fall under gravity.
Philosophical question to keep you awake on a long night flight - are contrails black or white in the dark?