Does anyone else noticed অথবা have any idea on why almost everyone is this episode is wearing purple? Ted, Marshal, Lily, Kevin, Norah, even her father, people at the stadium, at the bar. I can't find anything about it anywhere.

Yeah, I noticed it too. That and the contrast with Robin's wearing green.
I'm not saying it is in any way intentional in "Tick, Tick, Tick" - it most likely is an aesthetic choice - but there is an interesting reference to the color purple in "Desperation Day" (season 6, episode 16): Robin and her co-workers are all wearing purple as a protest against Valentine's দিন and the traditional red and pink; Bev, one of Robin's co-workers, says that she uses রঙ to express emotions and that purple is for "pride".

And the green dress Robin's wearing is almost the exact same hue as the suit Barney's wearing on Saint Patrick's দিন in "No Tomorrow" (season 3, episode 12) where they discuss the whole "do bad, good things happen" theory and Ted considers cheating with a married woman.

Again, I'm not saying it's intentional, probably just a coincidence!