Ha, gmail stopped sending me updates.
Thank you both for pushing it forward. I have less time for now.
This is great @BluePrintRandom!
I’d love to test it too on a landscape when I can.
Hello there. Any news on the library and colorisation based on slope?
This is still important for me 
Thank you!
step 1 for each quad find it’s adjacent neighbors, also collect what edge loops belong to each vertex
step 2 calculate the difference in elevation and use this to color the edge_loops vertex color that the vertex ‘owns’
Ah, well, thank you @BluePrintRandom
Unfortunately, I’m not a coder but an artist. Even if I understand the “python for dummies”, I have no idea how to define “quads”, “collect edges loops”.
Maybe you or someone could adapt some python from https://github.com/search?q=slope+path+A* to get a BPY script?
If you can’t, I still wanted to thank you for pointing it in the right direction with the right algorithm names and providing links to explain.
EDIT:
The only usable solution I found so far … https://www.youtube.com/watch?v=1c6UmnJDdV0
well …
Adding a blender resource to add road on heightmap (but not path finding it): https://beamng.com/threads/tutorial-adding-heightmap-roads-with-blender-v2.39903/
Pretty old though
there is an addon to project an helix curve onto an object shape
might work better then the skrinkwrap modifier
old addon and to find this script
but still does not check the slope of the path !
happy bl
Thanks @RickyBlender 
You’re right, doesn’t check the slope.
Hmmm, do you know an addon to get iso lines? You see what I mean?
Lines which are at same level in topologic maps.
Because if you can set the elevation step between 2 isolines, and have a segment (fixed distance to respect the slope angle depending on 2 isolines distance) maybe, by (programmatically) searching to link isolines with this segment, it could simply work.
Here’s an example:
If you have a 5.8m elevation between 2 isolines and you have a segment measuring 100m that starts at one isoline and join the next isoline above (from one vertex of the segment to the other without changing length), you basically draw a straight path with a 5.8% (3.3°) slope.
You might start from this new point to try to join the isoline above or continue on the isoline (take a curve for example).
Of course, it means that you might have a ditch under the segment, so you might add some gravel under your road or a bridge, or avoid this route.
2nd idea, an isoline with an angle (slope), starting at point A and ending at extremum (highest point before going down and back to A).
Like a series of planes starting at the top intersection with the landscape:
From there you have a boolean and only keep one side, so you could go helix or hair pin curves on one side of the mountain or mix both.
OK, you might have with some negative slopes. But they exist in life too.
No idea how to implement this in code though 
What’s the addon?
cant find this isoline thing !
saw something this year but can’t remember for now where !
has something to do do with contour lines may be or photogrammetry
if i remember will be back
may be open a new thread for this isoline function of Z thing
happy bl
That thread seems to be well beyond my meager Blender skills
And you’re way beyond my artistic level on Blender,
Like @RickyBlender and @BluePrintRandom, your portfolio is impressive!
you could also open up a thread on blenderstack
might get lucky !
happy bl
can you upload a sample blend file with a typical 3D shape
to add some isolines
for a Z value you could set a circle at that level Z and shrinkwrap it
after doing several isolines you could use the min path in blender
between 2 levels
happy bl
Here’s a blend file. Circle’s not happy to shrinkwrap around it though.
StackOverflow in general is not my friend. Too many patronizing, and sometimes yelling, people.
did a quick test here
and seems to work fine
and horizontal
note
i’m stuck on 2.9 only and 2.79
no later version
now depends what the ultimat goal is here
you can make as many iso line at different levels Z
but after that not certain what else you need
happy bl
In a different domain: If your photo colors matter, I just PR a github Blender add-on to help you calibrate images with a color checker : Color Checker Calibrator - #5 by MagnussonProductions
Since the project is almost one year old and it has a bug, if you know code enough, you might be able to help.