A solar water heating system works on Three main principles:

1.Black objects absorb heat.
2.Heat Rises
3.Thermosiphon

BLACK OBJECTS ABSORB HEAT

It is a known fact that black absorbs heat. The reason for this is that black objects absorb all the different color rays from the sun.

In the diagram below we see the sun's rays penetrating a prism which splits the rays into different colors. Therefore whatever color the human eye sees is what color ray that object absorbs.

EG: a red object will reflect or emit most of the red rays from the sun.

So if all the sun's different color rays are absorbed by black objects then that object has a greater potential to attract more energy and therefore can convert more energy into heat.