There's a way around that too, I think it something to do with vitamin B and a couple aspirin. But yea, you dilute your piss, then take a few vitamins/aspirin and it gives your piss some substance and color. So they stick their little test swab in the diluted piss, it of course comes up clean, and the poor lab tech guy would never even think twice about it.
I think they called it substitution. But who cares really? If it's that important just give it a break for a few weeks and get some excecise.