Mr. Toast is right. The EMIT/EIA 50 ng/ml cutoff is functionally equivalent to the GC/MS 15 ng/ml cutoff. The GC/MS is not more likely to come up positive.

EDIT:
Also, the lightness/darkness of your test line is not indicative of more/less drug metabolite concentration. If you can see the line, you're negative.

What the lightness/darkness really indicates are the pre-determined cutoff levels. The line on a marijuana test is very light because it's only looking for a small concentration (50 ng/ml)... whereas a drug that needs a higher concentration (say, 1000 ng/ml) will usually show a relatively darker test line when you pass.

You can drop a 5-panel home test in clean urine and the THC line will almost always come up far more pale than the other drugs.