Coolant get dark while driving.

Hi,
I do have "DD15, 475hp 2014 Cascadia Evo 13 speed manual 1012xxx km)

Here is the problem ("not sure")
Truck coolant turn red on overnight parking. (but i see black layer sits on the bottom of coolant pan)
Just after driving it turns black. (carbon)
There is no lose of coolant, bad fuel millage, white/black smoke.
Oil is not mixing up with coolant, or vise versa.
Fuel is not mixing up with coolant/Oil or vise versa.
Gasket is ok, no oil leakage.
Pressure test came good.
Freightliner thought it is exhaust gas mixing up with coolant, they wanted to open head gasket back in feb 2017 i said no nothing is wrong, and i drove since then no issues.
Ony guy thought it it could be bad EGR cooler!!

Any idea????
Thanks.

 

Yes when i took it to freightliner thats what they said. what does that means? crack on engine block? or what