Well Ive got over 100 stars now but i havent a clue how you get to do the purple coin challenges. Ive heard you have to look out for a purple comet but I havent seen a purple one. Am I missing something???

Please help me :(

Well I can't help (I get mine tomorrow) but I'll direct you to a video with all of the challenges.

But it features a major spoiler.

You know what happens when you get 120 stars right. (Say yes or no)

no, but if you put it in black I will read it anyways. I don't care about spoilers. I think it has something to do with how you can play the 2nd time around.

I think you may need to beat bowser to get the purple coin challenges...

Yes i think the purple comet appears after you defeat Bowser at the centre of the universe. (i can't remember entirely but that seem about the place it started for me...I did the final level after 61 stars)

Good luck on the "Luigi's purple coins" mission

Yeah, you need to beat bowser and get the plot ending first.

Then purple comets will appear and you can do 100 coin challenges.

When i was doing the purple coins on Freezeflame i kept falling in the icy water, I thought at first there was a glitch because the screen does a weird zig-zag thing when you do, but i vaguely remember a similar thing in SM64 for icy water.
(btw it isn't just on the purple coins thing, i just noticed it first doing that because i was falling in a lot more)

Yah it's just a distortion effect when you fall in.

Aye i thought so... just seemed weird at first....I think i wa just frustrated at the time because i kept falling in the water, and again like SM64 it takes me ages to get used to jumping out of water (gitty plumber keeps going under platforms i want to get on) so i was blaming my inexperiance on a glitch until i remembered similar effects in 64.

I dont know if I had to do this, but I just talked to the Luma outside of the first galaxy and had him transport the comet to whatever planet I wanted. I have 120 stars. >.<