I think that the colour of the band of the Duchess of Cambridge's engagement ring in the photo posted by Nellie is a result of a series of issues to do with light and how one colour influences another in photography.
At times a similar brassy hue could be seen when Diana wore the ring as seen here in August 1990.
I don't believe that the Duchess has been photographed wearing a replica. Even if a replica had been made it would be done in white gold to resemble the original. It is the sapphire and diamonds plus its provenance which makes the ring valuable.
I am convinced that the band is white gold which normally throws a greyish tinge when seen next to yellow gold and in particular the diamond eternity band.
I noticed when looking for photos of the Duchess and the ring that photos taken when the engagement was announced show wear marks characteristic of a ring which has been worn over a long period. The colour also makes me now think that it was not rhodium plated when given to the Duchess. I still think the setting is probably palladium.
When the Duchess has been photographed in the past the greyish hue is noticeable against her yellow gold wedding ring. If she had a white gold wedding ring, the colour would not be so discernible.
In December 2015
In June 2014
In April 2011
The same characteristic colour can be seen in photos of Princess Diana