Different colours of light do not penetrate or go through water equally well. Green penetrates deeply. Red light is absorbed rapidly in the pond by water molecules and algae.
Plants use more of the red light in photosynthesis than any other colour.
Blue is more scattered and reflected by water. Clear, clean water is blue. Blue water in a pond may mean that the water is sterile. The colour of water is changed by what is dissolved in it. Living organic matter gives water a greenish tinge.