While poking around the files I figured out where the overlay was and also how to package up a change for it into a mod. I darkened up the center's color a little and that was enough for the colors to contrast from each other and the ground enough to make it useable. They then suggested that a blue might also be a good alternative color to contrast with them, so I put together a version with that. Since it works well enough for them I figured I'd share it here in case anyone else is having similar issues. Or in case they simply want to try a new middle color for it.
If someone does have an issue with the visibility of the overlays themselves and can think of another color/shade/whatever that would work for them, then feel free to let me know. I'd be glad to see what I can do to help.
To recap:
A new shade and a new color for the center square overlay of the roboport coverage (aka. the logistics robot coverage)
--one slightly darker shade of orange
--one shade of blue
Downloads after the demo screenshots I took. You only need to download one or the other, whichever color you prefer.