The most are thinking that YX is possessing through someone just because his eyes are different in the beginning of the intro.
Let me state this clear he is looking at a purple star that is reflecting to his eyes. The purple star is foreshadowing the whole series like the intro starts with BBS.
YX probably knows what will happen cause of this star.

I have been thinking and I think that the purple star has something to do with MoM.
It is possible that the star is possessing Xehanort because he got beams with purple light.
YX could have seen the truth in the purple star and knew what to do.

So for the conclusion MoM is giving Young Xehanort knowledge what will happen in the KH series.

I saw the whole purple star as more symbolic than having a direct meaning, especially when you consider the previous entries openings. However, whilst stars represent worlds, they also represent people, what would be interesting if he saw his own star and thus learned of the events that will happen. 

I think the star was symbolic as well. The color purple in the KH series represents darkness so I think that it was just meant to represent Xehanort's fixation on darkness and how that led him to do all the things he did in the KH series.