I reckon one way to start understanding why to stop it the extraction when it is too blonde is split the shot into two. Put one cup under for the first 10-15 secs, then throw another cup under for the last half. Then try them, and I reckon youll start learning colour wise, what part your dont necessarily want.
One thing to look for is not only the colour, but how its coming out off the spouts. If it starts lookng watery its usually a good sign youre over-extracting and youre gonna end up with bitter-fest.
Let youre mouth do the looking ;)