What a neat quilt.
https://www.quiltingcompany.com/welc...quilting-blog/
Assuming of course this is the one you are asking about, but I think it is as it references McCalls.
I have not made this quilt but it looks like it will be pretty easy to modify to make it larger by adding more FG units that make the trees and/or majestic mountains blocks. In my mind part of the patterns interest is the very muted monotone color palette. I bet you could add some green but I would keep it in the same tone/hue so use a very muted sage shade of green. You would need to audition the color to make sure your eye isn't drawn right to that spot because it has a pop of color, unless of course, that is your intent.
I googled images of it and got lots of hits.
https://www.google.com/search?q=mist...w=1124&bih=761
It looks like others have also put in pops of green. Here is one:
https://www.pinterest.com/pin/460070918165059736/
BTW I LOL at your avatar.