The proper vitamin for urine color is B2 (Riboflavin), not B12.
However, it must be understood that with creatinine and specific gravity checks being the norm in recent years, color became the least aspect in determining an overly-diluted urine sample. Even samples with a nice yellow color to them can still be ruled "diluted" due to low S/G and creatinine readings. Therefore, its more important to take steps to maintain satisfactory S/G and creatinine levels when doing dilution techniques.

With your retest being tomorrow, it would be too late for you to start loading up on creatine, as it takes 2-3 days for the body to convert creatine into creatinine in order for it to be present in urine. Therefore, you should seriously consider subbing for the retest.