Youre worrying too much about faint lines when there really is no need to worry.
There is no meaning attributed to the intensity or width of the line. A line in the "T" region (drug), even a faint one, is a negative.

Also, the vitamin to use to color your urine is B2 (Riboflavin), not B12 (see the Dilution sticky). Keep in mind though that color is the least aspect in determining an overly diluted sample. Even samples with a nice yellow color to them can still fall below the acceptable limits for creatinine and specific gravity, and thus being ruled "too-dilute".