Here I found an article from the University of Miami Biology Department. I believe diet and medications can also effect the color of the buns urine.
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The Color of Urine Healthy rabbits excrete excess calcium salts via the renal system, and this can give the urine a chalky or opaque appearance. The urine will often dry to a white, chalky residue. Unless the residue is thick, pasty, and the color of mustard powder, this is normal, and should not be considered “sludge.”
Normal rabbit urine is usually pale yellow in color, but upon exposure to the atmosphere, compounds in the urine may oxidize to darker yellow, orange, red, or even dark brown. This isn’t unusual, and–by itself–is not necessarily a sign of a health problem. Blood in the urine, unless it is from a hemorraghing uterus or very serious problem, is usually not readily visible to the naked eye. Test strips are available at most pharmacies that will tell you whether there is blood in the urine or not, but your vet is the best judge of whether your bunny’s urine is normal.
Urine that is very dark immediately when it emerges may indicate that the bunny is dehydrated, and should receive more water, either by mouth or–in more serious cases–via administration of subcutaneous Lactated Ringer Solution.