In The Phantom of The Opera (2004), present days are in black & white:
Whereas past days are in color:
Why is that?
The quality of the two also looks very different.
It appears to me that the footage from the "present" (1919) is made to look grainy and poor quality as well as black and white, this is very similar to how films from that time looked.
Dustin Putman had the same thoughts in his review:
..alternating back and forth sparsely, unobtrusively, and at just the right times. These sequences are shot in gritty black-and-white, mesmerizingly evoking the look and feel of silent pictures from the 1910s time period in which they are set.
This would suggest that the filmmakers are trying to suggest that the footage from 1919 is shot on a camera from that era and is "actual" footage.
Where as the bulk of the film is portrayed as a flashback, with the quality being higher as though it is being remembered and not just played back on a film.
This also serves to make it apparent the time frame that the current scene is in.
As with my answer to your previous question about Phantom of the Opera, I should start by pointing out that I've only seen the movie once, but the musical several times.
Most of the story takes place in 1870, and in the few minutes at the start (and I believe the movie adds a scene at the end) Raoul is still alive, so it's not really present days (or he would have lived for 134 years after the main events).
I guessing they simply choose to make the main part (without having timed it, and as said I've only seen the movie - and thus the end scene - once, it's more than 95%) in colour, and do the parts in the future (related to the main part) in black and white to make it clear that it's at another time.