I posted something similar a few days ago- I took a test before I missed my period- it was faint. Then it got darker. Then 2 days after my missed period it was faint again. Now it's dark again. I don't think there is a straightforward answer... different brands will give you different results, and first morning urine has the highest concentration of HCG. Blood tests are the only way to get a quantitative measure (an actual number). I don't have that option because I won't have health insurance through my employer until 8/1, so I was just left guessing. In my case, I think what happened was that I had been drinking alot of water beforehand, and it diluted my urine. I have been using First Response Early Result tests each time and have gotten different colored lines each time, and none of them have really been as dark as the control line. They all have different amounts of dye in them and really are only designed to show if the hormone is present in your system or not, not how much of it is there.
If you go to the Countdown to Pregnancy website, there is an HPT gallery there and you can look by DPO and compare everyone else's results to yours- you will see there is a wide range!