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The Background layer is at the bottom of the Layer Editor. It cannot be drawn on.
The default color of the background layer is white, but it can be changed at any time. If you are looking for a way to make the subject “pop”, adjusting the background layer can help.
You can also make the background transparent; see Creating an alpha channel.
Changing the background color in Sketchbook Pro for desktop
If you want to change the color of the background of your composition, tap the circle on the Background layer to open the Color Editor and select a color.
Notice the Color Editor icon 
appears on the Background layer when you are changing its color.
Tap the circle again to stop editing the background color - note the icon will disappear once you've exited edit mode (the Color Editor will remain open).
Setting the background color in Sketchbook for mobile devices
If you want to change the color of the background of your composition, tap the circle within the bottom layer to access the Color Editor and select a color. 
