In any case, I hope the sporting tests were controlled for skin colour, because that does seem like a weighty factor when considering scoring bias.

For sure. In the case of the combat sports, outfit color was assigned randomly by the competition. In the Hagemann study, the outfits were alterted digitally so it was literally the same fighters. The goalie test which I linked to claims to use the same person just with different jerseys. For English football it seems unlikely that skin color and jersey color had any correlation but of course it wasn't explicitly controlled.

EDIT: Though it occurs to me that red could have different effects depending on the skin tone of the competitors (helps darker contestants, hurts lighter ones or something) and that certainly wasn't controlled for in any of the studies.