Using egg to coat roasts – endoring – was a common enough method in medieval cooking. The Königsberg MS includes instructions for doing this to chickens in several colours. They are not as detailed as the parallel ones found in Meister Hans (#179-184), but nonetheless interesting.
[] Willthu weiﬂ machen gebrottenn Huner:
If you want to make roast chickens white
Mix egg whites and a little flour and pour it over the chickens, then place them back by the fire.
[] Zu grunnen H¸nerenn:
For green chickens
Take parsley, pound it in a mortar, add eggs and a little flour and drizzle it on the chicken, then place them back by the fire
[] Zu swarczenn Huner:
For black chickens
Take gingerbread (Leckcochenn) and brown it until it is black as Pfefferbrott, grind it finely in a mortar and add eggs. Pass it through a cloth together and pour it over the chickens. When they are roasted they should have cloves stuck in them.
[] Salczen zu weyssen Hunern:
Sauce with white chickens
Take raisins, pound them small, take the best of wine and pass this through a cloth together with good spices.
[] Salczen zu rotten Hunerenn:
Sauce with red chickens
Then take parsley and vinegar and mix it well.
[] Sawarzes Huneres:
(Sauce with) Black chicken
Take almond and raisins in equal amount and pass it through a cloth with vinegar and wine.
The fact that the manuscript includes instructions for green chickens, but a sauce for red suggests that there was some transmission loss along the way into it. The technique itself is fairly basic. I have not been able to make it work with strong colours, but I have not experimented with it much either.
The Königsberg MS was preserved in the archive of the Teutonic Order in Königsberg (today Kaliningrad) in Baltic East Prussia, though its language suggests that it belongs to a Central or South German background. It is not associated with any name that I am aware of and is dated to the late 15th century purely on the scribal hand. The recipe types match South German sources of that time. It was published in Gollub (Hg.): Aus der Küche der deutschen Ordensritter. in: Prussia 31 (1935) pp. 118-124.