[bugmesome]

No idea about colour (I thought they dyed the fuel-excise-exempt stuff red or something, but could be mis-remembering how that works?)

But as an experiment... I just opened the filler cap on my TDCi (was ~75% full) and no psssh. I've heard it on petrol but can't recall ever on diesel cars.

Could be variations in diesel fuel, for example (depending on where you are) it could be winter-blend diesel. But from what I've heard it shoudn't make that much difference.