FIrst and foremost, if you notice a "slight lightening" in the middle, you need to figure out exactly what is causing it before taking any steps to "correct" it... it's not always a lack of nutrients. And only an overdose of nitrogen will show as dark green leaves - overdoses of other nutrients will show completely different symptoms! Similarly, if you've got yellowing leaves, it's not guaranteed that she's hungry for nitrogen... it could be any number of things that is going wrong... Always a good idea, if you're unsure, to come here and ask the question!
In this case, since you're growing an autoflower, you've been giving her FAR too much food for several weeks at least. Your EC shouldn't be higher than 1 or 1 point 2 at this stage of the game (and for the rest of your grow). She's needing a lot less nitrogen and a lot more potassium and phosphorus than she's getting, as well... I don't know your particular brand of nutrients so I can't give you specific suggestions - but get that EC down (I would probably flush her out until the EC of the runoff is about 1 and go from there) ... when you feed again, make sure the ratio of NPK is 1-2-3 ... and add some calmag as well...
Good luck!