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.
Can show the approximate code?
No, I think you should research the things I mentioned and give it a shot yourself first. I (and everyone else on the forums) will be available for help if you're not able to figure some of it out yourself, but we're not going to do all of it for you.