If it was getting darker and then lighter the following days one of two things have happened 1) you missed your surge and yes the OPKs would then starting lightening or 2) you were gearing up to ovulated but then you didn't. You could continue to take OPKs on the chance that the latter has happened (I have totally done this eventhough I did find out that I have in fact ovulated).
OPKs test for Luteinizing Hormone (LH) in your urine. You almost always have some, so at any point in your cycle, you could have very faint lines. This is why it's positive when the test line is as dark or darker than the control line.
So yes, after you ovulate, the lines go back to being light again.
“Life is not orderly. No matter how we try to make it so, right in the middle of it lose a leg, fall in love, drop a jar of applesauce.” - Natalie Goldberg
lH is always in your system. It will spike and then fade out. If you have multiple surges, it typically happens within a few days. When do you normally ovulate?