I do that using IrfanView, as noted. (If you’re not familiar with it, IV is a very fast, light, but reasonably powerful image file viewer with some image enhancement options.) But when one starts with a white road on a beige background, there’s only so much one can do, esp. if one doesn’t want to use up all their toner on nearly “black” lakes and parks.
I use IV as well and find it more useful than PP.
What level of detail do you want?
Is this for street or terrain?
Just thought of something...
If you could use something like photoshop elements you can convert the map to monochrome (greyscale or B&W) then really push the contrast.
If you aren’t so concerned with colors, and just the roads that might work. YOu might even try inverting the colors.
I use PhotoShop to darken the color or even change the color.
Doesn’t matter if it’s a map or an photo.
IV has it’s limits. Because it’s free.