If is selected, you make general modifications on the newsletter.
If you change the « External Background » color, it will change everywhere in the newsletter. The « Background Color » changes the block background color everywhere in the newsletter.
If you want to change only one block color, select it and, in the style tab on the right, move the button.
next to the settings, means the color is customed for one block only.
Click on it to remove the settings to default.