Illustrator :: Color Differences Between High And Low Resolution Images In Pdfs

OK, I'm creating a tradeshow graphic for a client. I designed the whole thing in Illlustrator CS6. But for the proof, I used a low res image of the sky (before we purchased it). Everyone was happy. It was a low-res RGB image imported into Illustrator, then the whole thing was exported as a pdf.
 
Once they approved it, I purchased the high res image. RGB. Same image... just high res. I popped it into the Illustrator file and exported it in exactly the same way. But now, the image looks much more purple. The low res pdf showed it as much brighter, lighter blue.
 
My client prefers the lighter blue. When I look at the two images in Photoshop, they look the same (in terms of color). should I be worred? Why is there such a color difference?