not completely satisfied with sven's method, working on some pictures
but not any, depending on the shades.
I used to do it in CMYK a long time ago with photoshop by just pushing
the image to CMYK and inverting the K curve.
I guess something might be done with decompose color filter or so. But
the quicker/best average i've found is just adding a mask based on the
layer (L from Lab or N form CMYK could be used too), and play with the
curve to adapt to the shades+refine with brush on mask. It takes few
seconds if the picture is not too complex. Then put a white layer below
and it's quite done.
But i'm not completely satisfied with it.
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