1. Create a new document with white or black background color set.
2. Use Scripter to run:
```
lang=python
from krita import Krita
from PyQt5.QtGui import QColor
kt = Krita.instance()
doc = kt.activeDocument()
test_color = QColor(128, 128, 128, 255) # change as needed
old_background = doc.backGroundColor()
print("Old background: {}".format(old_background.getRgb()))
doc.setBackGroundColor(test_color)
new_background = doc.backGroundColor()
print("New background {}".format(new_background.getRgb()))
```
3. Change test_color to see canvas update accordingly.