On August 14, Emily Blunt was spotted on the set of "The Devil Wears Prada" sequel sporting a dramatically different look. She's now rocking a platinum blonde extended bob with dark roots.
This is a striking contrast to the copper-colored hair of her character Emily Charlton, which she was seen wearing on set just two weeks earlier.

Source:
https://www.vogue.com/
There's likely a reason behind this image overhaul. According to plot details for the sequel, Emily Charlton now runs a premium clothing brand, while Miranda Priestly is trying to save her magazine from bankruptcy and has to humble herself before her former assistant. In this context, the radical hair transformation could symbolize a complete rejection of her past self and a power play.

Source:
Vogue, IMDb