Oceans and lakes are blue, but if you fill a cup with water, it’s clear! What’s going on?
When we see color, we’re seeing the energy of light that bounces off an object. A basketball is orange because orange light is reflected off it, toward our eyes. The color orange is just a type of light energy, and our brains understand it as color. Other colors of light, like blue, green and purple, are absorbed by the basketball, and we don’t see them. If you shined a blue light at a basketball, it would actually look black because all the blue color is being absorbed, and there is no orange light to reflect back to our eyes.
Water is pretty interesting because it doesn’t really absorb or reflect much color at all! Light passes through it pretty easily.
There aren’t any huge, gaping holes in water that light can just go through, like a car going through a tunnel. Water is jam-packed with tiny microscopic particles. One group of those particles, the electrons, is responsible for causing color (or no color at all!) Electrons are tiny active particles that travel around the atoms that make up water and all substances. The electrons in water aren’t too fond of the energy of visible light. They prefer other types of energy like ultraviolet and infrared, which get absorbed. We can’t see those types of energy with out eyes. Since waters’ electrons aren’t that interested in the light or energy we can see, the light travels by them without much of it being absorbed.