Hi ready4baby#2 the OPK's that I use have a control line and then the test line... The control line will always some up dark and the test line needs to be 'as dark' or 'darker' than the control line for it to be considered a negative.
The good news is that I start seeing a test line come up a few days in a row before it actually gets darker. I usually have 2 days in a row which aer considred positive and the rest are unfortunately negative HTH
I reccomend going by the instructions for your particular test type though