All rabbits normally excrete excess calcium and oxalate salts via the urinary tract, and the residue of normal urine will often appear "chalky." However, when excessive amounts of calcium/oxalate salts precipitate in the urinary tract, they sometimes manifest as a thick, curry-colored "sludge" that sometimes has a consistency as thick as toothpaste. This can be very painful in the bladder and when it is passed, and sludge buildup can cause urine leakage and incontinence.
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.
this is what i found on a website:
http://www.bio.miami.edu/hare/urinary.html