Black tires only?

Could be many reasons for that rule. Sometimes the host is looking for a race where the cars look realistic. Sometimes it's to avoid having cars with skinny and wagon wheels that typically reduce friction and give cars an advantage. 

Hi there, all the best to you on your diecast journey!

Black Tyres, yeah, I think some race promoters want the cars to look real, fair enough.

For me, as a racer, I would like to be able to run any freely available colour. But that is not about the colour itself, it is the quality of the wheel that matters. And some times that can take a few sets of farmed wheels to find the right set. (There is also a difference in material making up some of the wheels)

I do understand Blue Line Racings other point. I take my hat off to those chasing speed and obtaining those wagon wheels. For me, downunder, it is frustrating, as those wheels/cars are not freely available, rare in fact, and if you do find a casting, $20 and up. So, yeah, learning how to skill myself up to profile wheels myself (some time ago) to be able to do for applicable races was the go. All part of the hobby/sport.

Cheers Marc D

It's just a look thing and nothing else.

For us it's all about making the cars look realistic. If you wanted to paint the tyre part (don't need to do the rim) of a coloured wheel set cause they were fast, I'm fine with that for our races.

I've seen a few white tires with gold rims that would be sick on an all white bmw, Rick Ross style. Lol.

I'm not positive, but I think that there are different densities/properties depending on the color. White wheels do, definitely, sound and feel differently to me than black wheels. I imagine that it could make one faster than the other.