Can anyone help me with the Seratec O tests please?

I did a test this morning and it was sort of positive (I thought) but a tiny fraction lighter in colour - in the instructions it says it has to be same colour or darker. How do you determine if it is in fact the same or a fraction lighter? Neither my DH or I know 100% cause its looks so similar. I just did another test then (even though they say only do one a day ) and if it is lighter it has to be the most minute difference. I had a BT yesterday which apparently showed I was on the verge of O but the girl I spoke to today said that doesnt necessarily mean the next day. Transfer will be on Sunday if it is positive today, and I am having a scan tomorrow to confirm... but its sort of doing my head in LOL I keep going back to check it hasnt changed

How did any of you find these tests? Did yours seem to be very similar but not quite as dark? Or do you think it is probably a negative test? I feel like an absolute dope but I cant stop stressing about it