I too go with the very dark grey tyre (note UK spelling!!) colour (oops). In fact the actual tread portion can be an even lighter shade of grey, due to dust & wear. Go out & look at a few car tyre's (there we go again wth the spelling). Tyres are only black because a substance called carbon black is added as they are made. Over time, wear and ultraviolet rays tend to make them fade, hence the amount of stuff on the market that will 'return your car tyres to as new black' .
I've found that some matt black with a few drops of dark gray (or grey) added looks better than just black. Drybrush over the tread with a lighter shade and it makes all the difference.
As for that Humbrol colour, was it coal black?