I don’t know about black, except daylight was in short supply in December (and in shady forested areas) and tanks were probably wet too, so would certainly appear fairly dark especially early morning/late afternoon. Assuming they weren’t winter-camo’d of course.
But Battle of the Bulge aside, as for dark yellow there’s also the actual application of it. See Tankerken’s fantastic new post:
It’s highly unlikely that many of hundreds of German mechanics would all use the airbrush as diligently as this one for the camera…
…or that it would look the same on every tank, given the variations of what dilution was being used, what kind of diluter was being used, what was being over-sprayed, and how clean or dirty it was in the first place. In other words, F&!!#$!! yes it’s safe to say there were multiple shades of dunkelgelb as exhibited on German AFVs.