N overfert usually shows as a beautiful dark dark green in the plant which I have done personally.....the only other fert probs I had were 1. not using enough Cal Mag which caused some sort of MG def or something.....leaves were green but veins were yellow....stopped right away when I went to 50% Cal Mag with my nutes as suggested by my hydro guy (he told me to use 50 and 50 Cal Mag and nutes until I got to the top of the Cal Mag scale which I really havent' done yet...I use that all the way through flower) and when I went too low on the nutes...caused an iron def or something....the whole plant wilted down in about 24 hrs but came back w/higher nute level. None of these sound like your problem however.

keep us in the know....