Ooooh ok. Desaturate it in photoshop and then use photoshop to tint it via color balance. there is more than one way to desaturate in photoshop. one is through image > adjustments > desaturate but that's probably the one you tried that didn't give you accurate values. So try the other method which is image > mode >grey scale. then do image > mode >rgb to change it back and then go to image > adjustments >color balance and tint it to the color you want. if that works anyway. it's worth a shot since it only takes 15 seconds to do.
I see what you're talking about now. Unfortunately, since this is tinted in the 'old photo' way with many different colours, merely colourizing the picture doesn't come anywhere near what I want to do. I would have to lose the greens, reds, or yellows by doing that and I also lose the original colour of the pencil lines.
This one was adjusted from the original scan using curves and levels. Really, it's the puke-green splat on the ground that's really bugging me. Maybe I can single that part out and try.
Thank you so much for taking the time to write down the explanation. Otherwise I probablyw ould never have known what it was you meant me to do.