Most theories about where the Red Sea got its colorful name center on a red bacterial bloom that periodically appears on its surface, a directional association or its association with the Himyarites, rulers of Sheba. No historic record specifies the exact reason.
The most obvious answer of the three possible reasons is the trichodesmium bacterium, often called "sea sawdust," which floats near the top of water and has a reddish hue. This is the same bacterium that forms in oxygen-starved ocean water, sometimes causing a "red tide."
In the ancient world, red was one of several colors associated with directions. Black was north, red was south, green was west and white was east. This theory assumes that the Red Sea was considered to lie south of the ancient Near East.
The last theory is that the sea was named after the Himyarites, a people who once lived along its shores near Yemen and Ethiopia. The Himyarites were also associated with the royalty of Sheba. "Himyer" loosely translates to "red," and Sheba is very close to the ancient Hebrew "seba," or red. While it is uncertain why they may have been called the "red people," it's possible that this was related to the famous royal-purple dye obtained from the murex mollusk, which was actively harvested and processed in the area. Tyrian purple, one of this group of dyes, is closer to red, and it stains the hands of dyers.