I started by researching all I could on what they will be looking for on a drug test. It seems there are a few criteria: creatinine levels, specific gravity, and color of urine (this will help prevent red flags to be raised on the sample being diluted).
Actually, S/G and creatinine checks have become SOP in recent years, making sample "color" the least aspect in determining an overly-diluted urine sample.

Even samples with a nice yellow color to them can still be ruled "too-dilute" due to low S/G and creatinine levels.