Why does water always tend to be BLUE in color?

The water in our swimming pool receives light rays, it first absorbs red and ultraviolet light then yellow, green and purple colors, so the water remains blue because it is light with less absorption.

What does it depend on whether the color of the water is darker or lighter?

The deeper our pool is, the greater its capacity to absorb light and therefore the darker the color of the water we will see. In the case of shallow pools, the light absorption capacity is much lower and as a result we see the water more transparent. The time of day, the position of the clouds, the environment or the part of the world we are in also influences the color of the water in our swimming pool.