Plant turning light green, suggestions?

Hi all. Day 23 of flower. FFOF/castings/perlite. 6.5ph water, 1/2 strength FF big bloom, tiger bloom every other watering. 1 plant out of 4 seems to be turning light green. First three pics are side by side with the others. Last pic is 1 leaf from the bottom of the prob plant. The only leaf with a brown spot. All 4 plants treated exactly the same. 79-82 degrees 50%rh during light. I've looked at the leaf charts and to me it almost looks like phos def. or calcium. I don't think it's nitrogen, whole plant is turning, not from bottom up. I thought maybe a diff phenotype from the other 3? All 4 are vision seeds pure gelato. Plant appears to be healthy except for losing color, buds are the same size as others if not bigger. I've used 1/2 teaspoon of cal mag/gallon every watering except for the last two after I read it can lock out nitrogen if not actually needed. My tap water reads 125 ppm before calmag or nutes. Last watering was yesterday, water at 6.5 no cal mag or nutes. Not positive this is a problem but it doesn't look right to me. Next watering is probably tomorrow, right now I plan on adding 1/2 strength big bloom, tiger bloom, and 1/2 teaspoon of cal mag. (because it does sorta look like a cal deficiency). My first real grow, been going great so far with help from a lot of you.

 

Thanks. I think I will treat the other three the same as I have been, but cut the nutes down to a 1/4 for the light one. It hasn't lost any more color since the plain water yesterday, but I did notice that a few leaves have the most minor tip burn that you could imagine. Only a few leaves and barely there, but it's on the lightest leaves. I might even just go plain water on her one more time, then start her every third watering w/ nutes instead of every other. Will see how she does. As far as ppm, i've never measured run-off. Basically because when even when I water slow, I get run off out of the sides of the air-pot holes before I ever get runoff from the bottom. I figured measuring ppm runoff would be a waste of time because I'd be measuring a lot of straight nute water along with bottom runoff.

 

I do check ppm going in, the heaviest so far was 650 or so. I don't check ppm every time, just when I decide to add a little more. I didn't really start adding nutes 'til flower because I heard FFOF was good for the first 4-6 weeks. I did start big bloom at 1/2 strength at 3 weeks into veg, but that is the micro's, all organic,kelp and stuff. 0/0.5/0.7 . Started the Tiger bloom at less than 1/4 (300ppm?)every other water at 12/12 flip and was up to 1/2 strength after 3 weeks. Might have rushed a bit, specially the light green gal, I think you are right, she's a touch more sensitive.
I'll be honest, I was 50% convinced that it was a deficiency, and 50% convinced it was too much, lol. Didn't know what to do. Browsed all the leaf problem charts, looks sorta like this, but sorta looks like that too. Almost like this,but not quite, but also almost like that, hehe. Why can't real life problems look exactly like the example pictures?? Just another example of the Man trying to keep us down, I guess.
Will update if things take a turn for the worse, or if shows other signs. Been trying to keep my 'stupid' questions to myself, most of those are answered in search, but this one was bugging me.