What means 'I purple you'?

I read that Taehyung just made it up and that Armys thought it was cute, but what is the exact meaning? I read something about rainbows, but now that has me even more confused, lol.

I know it means something good so I will dare to say I purple you all

You mean Borahae. it's a combination of two Korean words: Purple (bora) and I love you (saranghae).

And the story you've heard - it was during a 2016 concert, V was at the intermission part where BTS speaks to ARMY and said (in Korean):

"Do you know what purple means? Purple is the last color of the rainbow colors. Purple means I will trust and love you for a long time… I just made it up. I wish I can see you for a long time. Just like the meaning of purple… we’ll always trust you and go up the stairs with you. You don’t need to help us all the time. You can now hold our hands and follow us now. We’ll go up really high. I’ll make it nice. Thank you so much for allowing us to have a fan meeting, in this kind of big place, ARMY I love you!"