Dan-San,
In the field, MvR wore the feldgrau (field-grey) Ulanka, which of course was not an aviator’s uniform. The Ulanka was the uniform worn by the Ulans.
Regarding your question about my sources, these are excellent books Dan-San. I recommend them very much.
1.
"Die feldgraue Uniformierung des deutschen Heeres 1907-1918", Band 1 und 2, by Jürgen Kraus, Verlag, 1999
and
2.
“Die deutsche Armee im Ersten Weltkrieg-Uniformierung und Ausrüstung – 1914 bis 1918" by Jürgen Kraus, Verlag
Blue skies, Dan-San
George