As much as I hate to admit it...most will benefit greatly by following Stink’s advice on "super urine". I would recommend using bladder urine fresh, cleanly handled glands and add about 10% glycerin (a preservative) to the mix.
UT Biology 101: Bladder urine is a sterile body fluid...if you have ANY bacteria in the bladder urine you are sick. Clean caught voided urine from all perfectly healthy individuals (man, women, cat, dog pig or zebra) contains 1000s if not 10s of thousands of bacteria per cc. These beasties live (colonize) the mucous membranes of the urethra and lower urinary tract; as the urine flows through, they wash off. Guess what these bacteria feast upon...urea. Guess what they release as they metabolize the urea...ammonia.
Now then I'm pretty sure that the urine you buy is far from clean caught...can't get the image out of my head...buzzard or stinky holding a cup in one hand and a bobcats tally whacker in another after he swabbed the exterior folds with an antiseptic rub...heck even better if it would have been a she-cat...but I digress. Commercial urine not only has normal UT microbiota but also loads of fecal organisms and feces...increase microbial load equals increase decomposition, equals increase production of ammonia. The ammonia once formed will "separated” from the liquid upon storage, even if sufficient microbial inhibitors were added.
But so what? When Mr or Mrs Kitty relieves them selves decomposition begins and continues until all the urea is broken down or all the water evaporates (add moisture and microbial action begins again). My point is that in nature urine breaks down but the critters still seem to be able to nose out the residuals.
Now then if one is using the urine in a lure formula one might want to QC the urine but for standard use most honest urines should work.
The darkness of the urine is related to the urea content. Urea is a breakdown product of protein / amino acid metabolism...heavy meat / blood diet and little water will make the darkest urine. Throw in fecal material and it gets darker still, albeit for different reasons.
I have drained many bladders from desert cats and have never encountered dark urine, unless it was from a waist snared cat and then the color was the result of hemorrhaging.
The best urine, then, will be that collected from the bladders to which glycerin (about 10% by volume) is added and then stored frozen or refrigerated until used. Glycerin above 6% is sufficiently antimicrobial to preserve otherwise "sterile" urine...all bets are off for pen caught urine and glycerin...way too many metabolic profiles among the fecal microbiota.
Hope this helps some.