Changing the colour of a popup Kivy - python-3.x

Question

I've got the below popup in one of my screens and I can't workout how to change the background colour of the popup, it just defaults to the Kivy standard grey. I've tried background_colour but it changed the entire screen behind instead.
def none_selected(self):
pop = Popup(title='Error',
content=Label(text='Please select at least one option', multiline=True,),
size_hint=(None, None), size=(250, 200))
pop.open()

Answer 1

If you are content to just change the background color of the Label portion of the Popup, you can just define your own Label subclass:
class MyLabel(Label):
pass
and in your 'kv':
<MyLabel>:
canvas.before:
Color:
rgba: 1,0,0,1
Rectangle:
pos: self.pos
size: self.size
Then, using MyLabel instead of Label in your Popup will give you a red background (but not for the title area of the Popup).
If you want to change the background color for the entire Popup, I think you will need to redefine the pre-defined style for the Popup. Again, create a subclass of Popup:
class MyPopup(Popup):
bg_color = ListProperty([0,0,0,1])
The bg_color will become the background color.
Now redefine the style:
<-MyPopup>:
_container: container
GridLayout:
padding: '12dp'
cols: 1
size_hint: None, None
pos: root.pos
size: root.size
Label:
canvas.before:
Color:
rgba: root.bg_color
Rectangle:
pos: self.pos
size: self.size
text: root.title
color: root.title_color
size_hint_y: None
height: self.texture_size[1] + dp(16)
text_size: self.width - dp(16), None
font_size: root.title_size
font_name: root.title_font
halign: root.title_align
Widget:
size_hint_y: None
height: dp(4)
canvas.before:
Color:
rgba: root.bg_color
Rectangle:
pos: self.pos
size: self.size
canvas:
Color:
rgba: root.separator_color
Rectangle:
pos: self.x, self.y + root.separator_height / 2.
size: self.width, root.separator_height
BoxLayout:
canvas.before:
Color:
rgba: root.bg_color
Rectangle:
pos: self.pos
size: self.size
id: container
The - at the start of the above kv, indicate that we are redefining the default style (most of the above kv is copied from the default style.kv). The use of canvas.before sets the background color. The MyPopup now has a bg_color property that you can set to whatever color you want, for example, to set the background to red:
def none_selected(self):
pop = MyPopup(title='Error',
content=Label(text='Please select at least one option', multiline=True,),
size_hint=(None, None), size=(250, 200), bg_color=[1,0,0,1])
pop.open()