I would have two seperate UIImageViews, one with the black and white version and one with color (and maybe a third with the red line.)
To generate the black and white version, I think you should look at CGFilters (I believe that's what they're called... I've experimented with them before but never used them in an actual app.)
Edit: oh, and I'd shift the bounds/frame around on whichever image is ontop when the slider is moving.