Well, I've been noticing when saving in a png format the picture comes in a darker color, no matter when I click on the transperent color it can't switch to another color, it's as if was lock on black. Step to produce this bug 1: Create a new image 2: Save new image as a png 3: Try to change the transperent color to white to avoid it from darkening.
Please check https://docs.krita.org/en/tutorials/saving-for-the-web.html