rainwarrior wrote:
I think it is usually chosen because it's the least commonly used saturated colour.
This. I know that at least in Allegro, they needed to use a saturated color because of different color depths (15-bit vs 16-bit vs 24-bit). Out of all eight fully saturated colors, magenta is the least likely to come up (cyan is also very unlikely, but somebody may end up using it as a sky color). That you can swap R and B and it still works is a nice side effect too.
Also it probably helps that Allegro was (and is) a really widespread library so you may find people using magenta out of custom (like I do).