Link to colour issue:
https://community.cesium.com/t/trouble-making-multi-coloured-polyline-clamped-to-ground/18567
I suspect my solution is to work out the terrain heights using Cesium and apply them to the data, then generate the polyline without clamp to ground. I’ve just not had time to learn how 
I have 30 years in underwater survey software development so I’m no stranger to datum problems.
From https://terra-dv.com/sandbox2.html
I made the attached screen grab. I made the line’s material width 100 so the problem would stand out. If I reduce it to only show 1 in 5 or so of the points the line segments are longer and display normally. If you look closely you’ll see that the segments are drawn vertically, not parallel to the ground. If I zoom in any further than that the line vanishes.
