What's the evidence that the colors were added later?

[oshfdk]

But the faintest traces, outside of those cases, are very faint but still visible.  Then why would only those two arms and three star outlines, specifically, fade much more tan other nearby traces?

Because these lines were still quite visible at the time of retracer 1, so Rt1 touched up everything else, but left these as they were. Then Rt2 skipped this folio altogether, and when Rt3 got it, only traces left by Rt1 were still visible, but not the original lines, so only Rt1 lines got retraced to their present state.

I think I know the drill.