Just some thoughts:
The reason red is not allowed in unit insignias is the same reason some Luftwaffe units, coming from the West, had to make a rapid shift from red to yellow ID bands on wings, tails and undernose cowlings (JG2 as I recall), the risk of association with the other sides' national markings is too high. As large banded stripes however, while it's clearly too intense a shade to be functional in a natural environment, there is no risk of miss attribution of color as markings.
Similarly, late in the war, the U.S. as well as Germany had active night vision gear. It is one of the reasons for the shift to the final 'liebermuster' camouflage uniforms with the muted (light absorbent) colors that fade so badly they look almost albino-white today.
While normal, high contrast, camouflage cloth printing systems may work alright in daylight, at night, they form a horrible mess that is starkly differentiated. They literally often glow or form negative-image outlines totally different from the ambient scope images. So too does painting out -anything- white on an AFV have benefit to those trying to control signatures on
vehicles. If you've ever seen someone light up a cigarette on NVDs of some kind, white paint is about a quarter as bright but more persistent on the
phosphor display.
The changing Dunkelgelb has an easier meaning to ascertain: it's winter or the beginning of spring and the 'tan' shade, at a distance, will simply be a 'light shadow' that separates and breaks down the total specular value of the rather dark intra-silouhette colors. When it's summer, they will likely go back to the darker shade or, more likely (if they are smart and given this is not the Ukraine), switch to two or more shades of green as countershade for the more verdant areas of Western Europe.
The red brown road wheels and lower hull areas I'm sure is more 'tincture' related, rather like MiG Pigments, in that, so long as you knock the edge off the saturated brilliance (i.e. taking a stopsign color down to a deep scarlet with some kind of light overspray or admixture of dark green or brown, even white if it's again, winter) it doesn't really matter what the -exact- starting shade is, it will quickly oxidize to a deeper tone or a more harmlessly pastel one, respectively.
I also think a lot of the examples you use particularly for the King Tiger belly armor and some patches on the Stug where there is no evident feathering are exactly what they look like: patches atop scratched up areas to act as corrosion control. Different from those areas on the interior which have been overcoated or on the exterior where red brown is achieved by dunkelgrunning the rotteoxid.
An important factor here is that the critical sector for a tank is the front and the top. Because those are the areas you want to keep facing a ground threat. And covered from air detection. The former is largely about tactics since there is little you can do to prevent detection once in combat. The latter is where the 'mobile bush' effect comes in, as so often in France after Normandy, where the sides and upper hulls are festooned with cut branches, a veritable garden cornucopia. Similarly, if you are hull down in a fighting position scrape or hidden under trees/houses, it's unlikely anyone will see your running gear while, in movement, it's the shadow zones that kill you. You want your entire vehicle to present the same light-reflectance levels across the entire surface area.
Indeed, in later camouflage systems like the Swedish Barracuda MCS, entire hanging panels of astroturf textured, curtain-like, assemblages depend from the main skirts and are solely dedicated to hiding the shadow-void of the darkened track areas.