Welcome to a magic wand exercise/demonstration. Choose the magic Wand tool (w).
Note in the options bar you can change the tolerance. What this means is the color range that it is going to select. The lower the tolerance (0-100 scale), the closer to the original pixel color (1 or 3 pixels wide where you click) that the wand will select across the entire image (all visible layers). A tolerance of 20 is about an average setting.
In this picture you'll see that the corners of the image are a little darker...
...so you might have to set a higher tolerance setting to get it all in one click.
Otherwise you can use 'add to selection' (as shown) to keep clicking with the magic wand to select the rest of the stucco colored wall. Do this to get any areas that still need to be selected whatever if your tolerance didn't get it all.
The magic wand isn't always magic. In most cases, it is usually tragic. But in an instance like this, it should be easy to make a selection out of the wall.
Once you've made your selection with the magic wand tool and got
it to where you've 'selected' the entire wall, you can now do
something like put it on it's own layer and change the color (you
don't have to put it on its own layer). As long as it is 'selected'
you can make adjustments to it (darker, different color, etc.)
without affecting the rest of the image.
You use selections when you want to make something happen to a
certain part of an image or layer. To be safe, just go ahead and
Layer: New: layer via copy or Ctrl J.
This puts that background wall onto it's own layer so you can 'work' on it without damaging the original layer/image.
More Tutorials: 
