By my experience, when it comes to rubber tires in model kits, car modelers get some good stuff: nice tread detail, well reproduced manufacturers marks...
By contrast, I've seen rubber tyres in some aircraft kits be nothing more than o-rings you could buy at a plumbing supply shop. So for us aviation modelers, rubber tires in a kit isn't always such a good thing.
As for tire colours, I usually paint the sides gunship grey with a touch of black mixed in and work my way into intermediate blue with a touch of neutral gray for the tread area.
By the way, the XB-70 wasn't the only aircraft with metal impregnated tires. The sidewalls of SR-71 tires were also silver coloured.