(edited step 4)

GIMP can select areas by color. You can select all white pixels on the image with one click. Maybe use a little tolerance to include your preferred shade of gray in the borders between white and black so it's not rough around the edges; it's a visible setting. Then add a layer mask based on that selection. White will then be transparent. You can save that as a PNG or put whatever color you want behind it with a fill of a other layer behind it.

Short answer:

1. GIMP

2. Select by color, click any white pixel

3. Menu: Layer, Mask, Add Layer Mask

4. Pick "selection" and check "invert selection" at the bottom.

Done!

Welcome from right down the road. I've done a map or two of my local area, but I'd like to do more in NC. Maybe we could collaborate. Have fun!

Here's an update. It's really minor stuff. I think I'm close to submitting this. I need to review the process, as it's been so long since I started!

Gallery Link

Here's an update. I added some detail. I'm stepping back to evaluate the entire map.

Click here for the larger gallery image.

All, much better. Thank you!