just updated Krita’s Color to Alpha filter on the development 2.7pre-alpha version ( already on Git master ) , so it can now be easily used for removing background from scanned sketches.
To find the feature, in the menu : Filter > Color > Color to alpha.
Algorithm differs from the one used in Gimp ( also named ‘Color to Alpha’ ) : Krita use CIE deltaE  function to calculate the difference between the image and the base color, so it is more flexible and configurable. You can choose to what extent you want to remove the background by changing threshold value.
Dmitry explain the filter’s algorithm on his blog :
What is the advantage compare to put your pencil artwork on a ‘mulitply’ layer blending mode ?
You can easily ‘lock’ the alpha of the layer, then color only your lines now. It’s a step I usually did in Gimp ( see on my pencil to digital painting tutorial ) and I’m happy to see the feature now in Krita too.
image & text copied from Dmitry blog
pencil artwork CC-By Deevad