Neptun,
That is a good question. natural rubber comes in a variety of shades and colors. Tires on the landing gear of WWI a/c, and i can assume trucks and cars as well, have been documented as being varying shades of grey, brown, and even pink! I'm not sure of your age, but when i was a kid in the 60's ican remember seeing baby buggies with white rubber wheels! That is always possible as well.
Your use of dark grey is fine, as are lighter shades of grey. The thing you want to avoid is the use of black, or very dark grey (almost black). This is the result of adding carbon black to rubber, and that did not become a common, accepted practice until after World War I.
Hope this helps,
Warren