Why were blue, yellow, black, green, and red chosen for the colors of the Olympic rings?

The colors were chosen by the IOC to signify the union of 5 continents: Australia, Africa, America, Asia and Europe. The colors L to R are Blue, yellow black, green and red, and were not meant to really mean anything, although these colors appear in many of the flags of countries on these continents. Some believe that blue = Europe, yellow is Asia, black is Africa, green is Australia and red is America. The white background signifies peace.

The Olympic flag is raised during the opening ceremonies of each Olympic Games, and lowered during the closing ceremonies.

The flag uses the ancient symbol of the Olympic Games (five interlocking rings); blue, yellow, black, green, and red respectively on a white field uncovered by Baron Pierre de Coubertin in 1892.
The flag was first raised during the VIIth Olympiad in Antwerp, Belgium in 1920.

It is fabled that every nation that has an Olympic team has at least one of their national colours in the Olympic flag.

The rings are said, thematically, to represent the habitable continents: Europe, Africa, Asia, Oceania, and the Americas
https://en.wikipedia.org/wiki/Olympic_flag