And a really cool thing about magenta is that it doesn’t actually exist as a frequency of light like yellow or green. Rather, it is the color our brain magically forms (!) when we see a combination of red and blue frequencies.
That color had to be distinct from other colors in that birds environment so the potential mates can find it easily, and it had to be sensed by the bird's visual system. So no! No one picked that color, it is what is should be, like the rest of the nature.