From past research, light Grey or pinkish grey is widely excepted as the probable color.
The B&W pictures offer a good amount of information on the color as well. IMHO you can tell that the tires are light Grey or pinkish grey in this picture. I don't personally believe they were black at all (in this specific picture).
1.) http://en.wikipedia.org/wiki/Fokker_Eindecker_fighters#/media/File:Fokker_M5K-MG_E5-15.jpg
a few more examples (compare tire color to dark leather boots and dark wood prop)
2.) http://wwimodeller.co.nz/wp-content/uploads/2012/03/Fokker-Eindecker-101-1200x802.jpg
3.) http://www.aerospaceweb.org/question/weapons/guns/fokker-eindecker.jpg