The difference in the colour of the pearls in the Empress' tiara is striking. Like you I had previously assumed that it was dim lighting which caused the pearls to look yellowish.
I suspect that some of the pearls are golden pearls which were initially found in waters surrounding Indonesia, formerly the Dutch East Indies.
After having looked more closely at the pearls, it seems to me that there is a pattern to where the yellowish pearls are used.
How I wish that the museum staff would have the tiara dismantled and the tarnish removed from the silver setting.