Some colours, principally the darker colours, will get heated more by sunlight than others. What this means is that that darker colurs get hotter! Black tubes will get seriously hot. This will heat the air in the tubes more, and according to Boyles Law, the air will expand. Thus pressure relief valves will be more advantageous in dark coloured tubes.
Of course, this years in colour is grey
As well is being one of the better colours for minimising the heating effect of sunlight, it suffers the least from the discolouration that hypalon is subject to when left exposed to the elements.
White tubes will absorb the least energy from sunlight, but it is not the most practical colour.