After the sink, you can see Rose at the Carpatia, and when Cal is looking for her, you can see her from the back with a blanket on her shoulders, and when Cal gets closer, you can see her already with the blanket on her head and a different blanket colour.
I think Ania is right on this one. I watched the movie right after I read this and if you look when Rose is in the lifeboat the blanket is a darker green. Then when she is on the Carpathia, it's a lighter color.
I don't have the movie in front of me to watch this scene, but is it possible the lighter color reflects the blanket being wet at first, and then gradually drying out in the wind? I'll have to see the scene for myself to see if this hypothesis applies...
You ever think those nice folks on the Carpathia gave her a new, dry, and slightly lighter color blanket when she hopped on board? No need to fret over that 'mistake.'
Actually Rose has the blanket on her head the whole time. When you first see her on the deck, the blanket is at the back of her head, then when she sees Cal, she pulls it all the way up to cover her face.