I don't think the problem with the engine wiring harness issue is a discoloration of the copper wire, it's that the insulation degrades, cracks, and falls off, allowing the wires to short out.
There are other places to check for failing insulation. You can use a utility knife and carefully slit open the black fabric covering of the wiring harness portion that runs across your firewall.
It will be real obvious.
It is characteristic of this problem to surface after the harness has been handled, in particular after a head removal and head gasket replacement.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".