How do i do this in PP?

[dr.batata]

I saw a presentation 2 years ago, where a class were showing their photos from past years.


The photos would show first in black and white, then gain color gradually.. each photo will do the same: black and white, then gains colors and move for the next photo to come in.. My Q: how could i do this? (other than using two versions of the same photo, one in black and white and one is colored and then make 'fade' appearance of the colored one! because there were so many photos, and i dont think that what the presenter did)
thank you..