Proposing an explanation for the changing colors of the sky is an example of a(n)

This would be an example of a hypothesis or a working explanation for a natural phenomena. The natural scientist first develops hypotheses to explain natural phenomena and then when they have been tested and are found to be universally correct then they become theories. This is like the theory of natural selection developed by Darwin and Wallace, so that from their careful observations of plants and animals in South America and Indonesia and years of study that followed, they proved that this mechanism had universal truth. A geologist is using many hypothesis as he works in the field say mapping the rocks identifying them and understanding their relationships and after many years studying them may arrive at a theory of their origin, development etc.

Ithink this is what you meant. the temperature at which a liquid begins to boil is called the boiling point. the boiling point of water is 212°f or 100°c.  and at that point will start to turn into steam (gas)