The attached arc has a pink color by assigning pink material. I don’t find the related C API how get its color data.
arc.skp (123.0 KB)
The color is applied to the individual edges of the Arc.
If you have an SUArcCurveRef you need to upcast that to SUCurveRef then use SUCurveGetEdges.
Note that it’s technically possible that each edge in the arc/curve have a separate color.
Arcs/Curves in SketchUp are not true entities - they are instead kind of meta type that describe the relationship between a set of entities.