Just putting out some thoughts here. I'm thinking you might want to clip/split the line into a segments that are on either side of the 3D object. You can transforming the line into a 3D object (3DForcer) and then sending it into the Clipper with the original 3D object as the Clipee. After you've determined which of the two segments for each line is going into the object and out of the object, you can then colour them depending on which format you're writing to and if it supports storing styles. Hope this helps.