Auto Union race numbers . . .

Is there any evidence that Auto Union race numbers were anything other than Red ?

Some cars demonstrated by Audi Tradition have Black numbers as do some models made by CMC.

Auto Unions - and Mercedes Benz - which ran at Donington Park in 1937 and 1938 carried black numbers; RAC regulations specified that race numbers could only be black on white or white on black. I presume that also applied to the car Hans Stuck ran at Shelsley Walsh - when Hans-Joachim demonstrated one there it had a black number and I've certainly seen black-numbered AUs at the FoS.

Black on white or black on silver?

I think a similar situation occurred in 1954/55 when the W196s had black numbers at Silverstone and Aintree

Auto Unions - and Mercedes Benz - which ran at Donington Park in 1937 and 1938 carried black numbers; RAC regulations specified that race numbers could only be black on white or white on black. I presume that also applied to the car Hans Stuck ran at Shelsley Walsh - when Hans-Joachim demonstrated one there it had a black number and I've certainly seen black-numbered AUs at the FoS.

 

How rigidly was that enforced? Bira's car had white numbers on blue in 1937.

How rigidly was that enforced? Bira's car had white numbers on blue in 1937.

And the Reusch/Seaman 1936 Donington winner had white numbers on the [presumably] red body.

I suggest this is a case of German thoroughness - complying with the letter of the regulations (just in case).  

https://m.youtube.co...h?v=M7y3QNUfQeM

The video shows Red numbers on both the Mercedes Benz and Auto Unions, about 34 seconds onwards.