I got this response from development today - I will try to make some use out of it early next week after the Memorial Day break, but I thought I would share it now
The SVL file format defines a fixed 255 color palette that the colors (defined as RGB colours) in the source linework are mapped to when converted to SVL. The mapping uses a 'shortest distance' algorithm to find the closest color between the RGB color provided and the RGB colors available in the fixed palette. In this case, 'distance' is the separation between the two colors defined in the RGB color cube.
This does mean the colors in SVL be similar to the original colors if the RGB values of the provided colors did not exactly match colors in the fixed palette.
Round tripping the data is possible due to the the SVD/SVL importers in TBC, though there are some caveats. Surface information from SVD files should look identical, but there is a small precision loss due to the way SVD files quantize vertex locations to 0.1mm (this makes the files substantially smaller). SVL files are little different, and the results depend on the three use cases SVL files are put to.
1. Site background linework map: Linework in these files should be visually identical, with the same point location quantization noted above. However, site background files are not tiled as for design background linework.
2. Design background linework map: Linework in these maps is broken down into tiles to allow the machine control system to only load linework around it to reduce memory pressure. Tiling means lines that cross tile boundaries are broken into multiple line segments, one per tile, so when re-imported the resulting linework may look visually the same, even though it has been broken up. It's worth noting breaking of lines occurs at tile boundaries (eg: 100x100 meter squares) and not at arbitrary intersections of linework. GCS/EW does have smarts to reassemble line segments for guidance to background linework - James would be the best person to comment on how well that works.
3. Alignment geometry: Centerline geometry is broken into tiles; the sequence of lines, arcs and curves are always retained as provided to the SVL conversion interface, with the exception of small tolerances (sub-millimeter) for curve fitting of curved geometry more complex than arcs. They retain precise stationing along the alignment at all locations.
SVD/SVL file formats were intended to be terminal formats optimized for use on the machine control system. Round tripping is possible to support centerline alignment import and to visually verify the design is correct, but they are not intended to be useful in the general design life cycle.
Hope that this helps - this is from the "Horses Mouth" so to speak so it is as detailed as yo are likely to get.
On the color subject - I would have to try this out to see what I get before commenting further
Happy Memorial Day - I am checking out tonight for a long weekend.
Alan