can anyone tell what deficiency this is?

I got gdp in rdwc and on my 3rd of veg. they dont seem to be looking as green as i want it to be. my water temp is around 68-69, ppm 690, ph is 5.2-58. feeding them ro water with gh 3 part mix. so it looks like it's light green where the viens are on the leaf. i hope these pic will be enough for you guys to help me. thanks in advance!

I'm not a hydro guy and I probably shouldn't even offer advice but RO water will pull some micros from your solution if I'm not mistaken. Perhaps you already know this. Just a guess but it looks like zinc def. could cause the symptoms you're explaining and the leaf cupping and twisting.

690 ppm sounds kinda low to me...but I'm not a hydro guy either.

Somewhere between Potassium and Magnesium. Maybe a touch too much calcium?

You won't get much more than guesses with pics so yellow. Try to take some under normal lighting if possible. For example, you say that the leaves are lighter on the veins but in the pics, it looks lighter between the veins.

690 is fine for hydro. i usually go up a quarter with every week and cap off at 1000 for the rest of the 8 weeks.