I am smiling because the color water was once topic of a debate in my second college physics class. To be specific, the color of any object is determined by light of the visible spectrum it doesn't absorb. The is applies to red and green apples too.
The professor challenged the classes overwhelming opinion that water is clear. He argued that water is no different than any other object in terms of color. It's color is the combination of the spectrum emanating from it. When one classmate argued that water was clear because you can see right through it he countered that water has both the property of transparency and of opacity. Water obstructs light even when it is perfectly pure. He argued that this why it is visible and that only a pure vacuum could be considered transparent.
In the end, I agreed then and remain in agreement that if the color property of an object is determined by the light spectrum we see when we look at it then the color of water must be blue.
