Ah, I had to look this up for the details but I knew the basics already.
And, basically, really early tires, 1800s to early 1900s, were white or off-white. Why? Simple, rubber is white. It isn't black, that's for sure
I had heard that tires manufacturers started adding carbon dust to their rubber, so they wouldn't show dirt as well. It turns out that's only part of the story. From a automobile page-
Joyrides | Blimps, whitewalls and radials -- How the tire industry developed
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Most of the early tires were white, the natural color of rubber. Some were tinted with a pigment and were light gray or tan or beige. But by World War I, they were all black. They were black because tire manufacturers began adding carbon black ash to the rubber recipe to enhance toughness and wearability.
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This turns out to not be completely true, at least as far as aero tires are concerned, because you can find light colored tires in photos all the way to the end of the war.
More, from another auto tire page-
Coker Tire Resource - History of the Tire Timeline
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1912 - Carbon black commercialized by:
*Diamond Rubber Co.
*B.F.Goodrich
1913-1917 - Tires
*4 or 6 Ply cotton, All-White or All-Black tires
*Conventional tire size 32x4, etc. straight side high pressure
*Flat base demountable rims
1917 - Last year for All-White Tires
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Again, this deals with auto tires and probably focuses on the US market.
But, that's the gist of it. Ya lean something new every day
To my eye, it looks like Germany used more white/light gray tires than other countries.
Either way, I can't pass up a fun detail like that!
Funny thing, I might need to use it, my wheel disks still show a 10 spoke outer pattern instead of eight 
I'll keep that in mind next time I do some WW1 bird...