Around when are the new colors expected to come out?

No date is announced. I would guess around the same time that the Lumia930 comes out (June), but that is entirely speculative :)
 
I own a white one, wanted a yellow one but now I'm totally obsessed with the green one. Guess I have to wait another 2 months to get a new, bright color :-(
 
How difficult is it to replace the back part of the phone with a different color? I have a black 1520.
 
How difficult is it to replace the back part of the phone with a different color? I have a black 1520.

As far as I know it is easy enough and I hope so because I have a black 1520 and I just bought a replacement yellow and white backs and sim/sd card trays.
 
It's easy but it does void the warranty, as you have to remove the "Do not remove" sticker to get to the screw.