Lol I was thinking of posting a question about this too! I'm amused by it because it's a way of thumbing your nose at convention by accentuating something you're expected to hide. I wouldn't do it personally though as armpit skin is sensitive and I think the dye would irriate mine.
Doesn't matter - because I am quite short in stature I'm usually at armpit level with a lot of people. Any armpit hair is revolting, particularly in summer with the sweat beads glistening in my face. Don't care what colour it is.