These are thin trails of condensation, formed by the water vapour rushing out from the engines of jet aircraft flying at high altitudes. They are not true clouds, but can remain in the sky for a long time, and grow into cirrus clouds. Light from both the sky and from clouds is sunlight which has been scattered. The water droplets in the cloud are much larger, and these larger particles scatter all of the colours of the spectrum by about the same amount, so white light from the sun emerges from the clouds still white. Sometimes, clouds have a yellowish or brownish tinge this is a sign of air pollution.
Condensation involves the release of latent heat.
This is the 'invisible' heat which a water droplet 'stores' when it changes from a liquid into a vapour. Its subsequent change of form again releases enough latent heat to make the damp parcel of air warmer than the air surrounding it. This allows the parcel of air to rise until all of the 'surplus' water vapour has condensed and all the latent heat has been released. Therefore, the main reason which stops clouds growing upwards is the end of the release of latent heat through the condensation process. There are two other factors which also play a role.