I restored some ww 1 aircraft and the colors in the zones exposed to the sunlight, dust, moisture and the like were much different from the same color preserved below an overlapping panel or between two ribs.
Much, much different
The white areas on the balilla were brown where exposed while they were off white under the shielding panels....
I am inclined to believe that the red Dr 1 was red, not that sort of NIVO :-)