What could this be. Pictures inside

[Mrkentee]

Could be condensation and gases (causing the color). Mine had the same, and it's disfunctional. When it was taken apart there was water in the egr valve.
If you're not losing coolant, and it's not all mucky in the piping, it's doubtful it's leaking. I wished mine was leaking causing cooling system pressure. But the cause of that was worst case scenario. Now I have a rebuilt engine with a new head.