Summer, I think some people like the colour of their stripe more than others
I don'k know how the colour is decided - that's somewhere in the depths of the programming used for the forum. I don't know if it's got any connection with the avatar name used though - if it did then that would mean the colour changed on a name change - that's something you might want to check with Lori. There's also internal numbers used to identify who is who. The programming is done by reference to those numbers so that the pages we see show whose post is who, who has liked each post, etc.
Your border color is hsl(-139, 20%, 80%) which is meaningless other than to a computer programmer