Here's how in PS.
1. Create a duplicate layer of the background image
2. Since this is a nice manmade shape - straight edges - it is doddle to use the pen tool and create a vector mask of the outline.
3. Layer->mask->rasterize to convert the vector mask to a normal mask
4. Invert the mask so that (select, mask Ctrl-i) the outside is masked not the shed.
5. Fiddle to your hearts content with the hue,sat (Ctrl-u) and or Curves (Ctrl-m)
HTH