I tend towards the lighter gray on my WWI Aviation tires. But I notice a lot of builders go for the dark tire black. So what is more accurate?

Not sure about the accuracy part, but Floquil has a color called "weathered black" that strikes me as the perfect balance between the two options you've mentioned. I'm just guessing here, but I don't think tire color has changed all that much, and if you look around at current examples (airshows like EAA offer tons), you'll probably see black, gray, and some in between.

Mike

The color of tires is impacted by the formula used to make them, chemicals primarily carbon, are added to the rubber to prevent dry rot, and increase durability during the "vulcanization" process. This can of course vary quite a bit from manufacturer to manufacturer and by country of origin. early "balloon" tires tend to be light gray in color, later tires take on the shades toward black. U.S. made tires tended to be darker than others, due to their formula. I know you are thinking of mostly WWI tires but an interesting example are the tires for operational SR-71's, they are very light gray almost white because instead of carbon, titanium is used to increase their temperature tolerance.

Since the mid 1950's tires tend to be black but for earlier subjects post WWI I use Testor's "Rubber" it's not quite black but not gray either.

As mentioned tire colors are based on the manufacturer and the processes they used. German tires seemed to me to be a bit grayish looking because of the raw materials they used. Because there was such a rubber shortage during WWII many companies (allied & axis) looked into using different materials to manufacture tires. Some tried various bases and supplements such as coal and synthetics. Aircraft operating off grass fields tended to be softer compounds whereas those who could operate off improved solid runways were harder, which allowed aircraft to carry heavier loads. Each tended to have a different color and tone so anything from a black, or dark brown to dark gray would be fairly accurate.

The comments about the amount of carbon black in the fabrication process are dead on. Another consideration was that permanent established airfields had ash landing strips. This had an effect on the tires. various colours have been noted. Red brown dirt ( with a large iron content) tending to turn light grey tires a light pinkish hue.

Most diorama builders were taught by Shep Paine's writings to carry the ground dirt up on to the boots and tires of subjects in a light coatings.