I have, got a ++ HPT before a ++ OPK tho. The OPK became gradually darker the closer my AF due date came but I didn't keep testing after the ++ HPT....
Sounds very promising for you!! If AF is due today you can test any time of day :)
Good luck , but it doesn't sound like you will be needing it!! ;)
xx
Look here http://www.peeonastick.com/opkhpt.html