OK - eyes closed all you non-Photoshop types? Then here goes...
Select inverse, so you now have all the pink paper, plus just the edges of the illustration selected.
Select inverse again and click the icon that creates a mask, which visually kills the paper background, doing most of the work, revealing the colour behind:
On the actual image (not the mask layer) lasso all the background area again, leaving a margin around the illustration, and delete: this will instantly kill any small specs or smudges you might miss.
Zoom right in and work on the mask layer to tidy up any anomalies (I use a fragmented or 'dissolve' brush to keep any marks as close to the pastel marks as possible).
As you go work your way round, you can swap to the clone tool (transferring to the image layer), to repair any edges that have unwanted pencil lines, or 'halos' of the previous background colour:
Next, lasso the shadow and, again on the image layer, adjust hue / saturation / brightness (or 'selective colour') until the shadow 'sits' properly on the background colour. On this example the original shadow's not too bad, as the purple is not a millions miles from the original pink paper, but if it was going onto yellow, it would be completely wrong:
Apply the mask.
Do any last minute tinkering (eg, I felt the hat was a bit washed out against the lilac, compared to the bears, so I altered the levels).
And then you're pretty much done!
I wish I was - I'm still working away on them every day at this end, with several more to go, and going slowly crazy.