yes Nitrogen.. i say because it looks like they flushed back green.. if that is the case you can fix it by adding a little extra grow nutes. most of the time myself i will just add something high in N like fish emulsion/coffee grounds/urine etc but until you become comfortable with your grow just stick to what you have.. this problem is no big deal (if N is the case) and can be fixed any time.. you don't have to give it a tone of N to quickly get it back.. just add a little extra grow next time you feed.. if you see the leaves flushing back green.. this is your problem .. but do it slow the plant has plenty of N in the other leaves and it would be better if the lower leaves got a little yellow then it would if you burn the plant.. as far as your PH.. you can get a pen on ebay delivered for under $10.. i hardly ever even PH anything anymore.. since i switched to 100% organic.. your PH doesn't have to be exact 6.5.. your plants don't appear to have any lock out.. they both look very nice.. you might want to consider harvesting the main cola on the one in the red pot when it's done and then lower the light.. and those lower tops will put on more weight
you can either or add to nutes or with this watering.. you can use a little this watering and if it flushes back put a little w/nutes next feeding also... it doesn't have to be early (but can be).. when the main cola is finished you can clip it and lower your light for another week or two and those tops will put on more weight.. you have some height on that main cola and it's still gunna grow a few more inches.. it's a kick ass job though first grow or not.. good for you