Hi Rod and Dont
Very definitely, as Dont suggests, with an edit program like Photoshop or Gimp (free).
Yes, you have to select colours like the green for forest and replace with white. The edit programs usually let you select a range of colour for generic replacement.
If you are having trouble with the small print, that is partly due to the original printing process, but can sometimes be improved by scanning at a higher resolution. If you want to be really fancy you can try a limited amount of blurring and thresholding to 'join the dots', but that is a bit messy.
As you may gather, I do this myself a lot. My topo maps stay at home: I only carry printed copies or photocopies in the field.