When Jack is leaving Rose after having dinner with her, he shakes her hand and gives her a note. When the note is in his hand it is yellow, when Rose opens it up it is white.
It is CLEARLY yellow when he HANDS it to her, when its being opened its kinda losing its yellowness. When she's reading it it is white, so it DUZ change.
That actually was a slipup; there was something on VH1 about it a while back. I think also that one of the notes was lined or something, too, it wasn't just a color difference. Kudos to you for your good observation!