the leafs that already are yellow wont turn back green, some leafs will more than likely continue to turn yellow as your grow progresses some people dont worry about it and say it is normal for a flowering plant, but i try to keep mine green through the whole grow cuz Uncle Ben said thats the thing to do lol
again ive never grown hydro but from what i hear should notice differnce in chages very shortly unlike soil which may take a few days. GL you really should look into getting a PPM meter if your going to grow hydro might be able to find a cheap one on ebay...