From peeonastick.com:
9. I took another test and the line is lighter. Does this mean anything? Should it get darker? Not necessarily. In general, the positive line will be darker when a greater amount of hCG is present in the urine, but not always. Some tests contain more or less dye (meaning there is just more or less color to "stick" to your hCG.) Some tests are more or less sensitive-- even 2 tests in the same box can be different. Also, your urine may have more or less hCG in it depending on what you ate or drank, or how long it's been since you last peed. Food itself does not cause false results, but can affect how much urine you have and how dilute it is. Eating salty chips and drinking caffeine all day will result in more concentrated pee, whereas gorging on watermelon and ginger ale will result in lots of very dilute pee. But an HPT is not meant to tell you "how" pregnant you are. It only tells you if you are pregnant or not. As long as there is a line in the results window, you are pregnant.
Some women experiencing early miscarriages do notice their lines getting fainter or disappearing entirely, but even if your line gets fainter, it is not a guarantee that you are miscarrying, or that anything is wrong. See FAQ 21 for normal levels of hCG and how to properly measure it.