The answer is part safety and part appearance.
A dark pool bottom and sides makes it hard to see the bottom. In some cases, it can look even "bottomless". This can be dangerous because someone may jump or dive in thinking that the water is deeper than it really is and injure themselves.
Also, a dark colored pool bottom and sides will make the water look dark and most people do not find "dark" water very attractive, instead thinking that the dark water looks more like a dirty pond than a crystal clear pool.
Some people want that "lagoon" look.
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