First GSC

That's called interveinal yellowing, it is a sign of magnesium deficiency. Usually there's enough present on tap water along with calcium. That you have a deficiency of it can be due a couple of reasons: first your tap water is too soft; second, you have a magnesium lockout, which happens when you have excess phosphorous (P) of potassium (K); third you are using reverse osmosis (RO) water or bottled water that's low on it; fourth, your pH is out of range preventing the absorption of mg. Before rushing to buy cal-mag or something similar (calcium with magnesium) or Epsom salts (magnesium with sulfur), get a pH meter and an EC/PPM meter, a cheap one will do, and check your water source, as long as it has an EC of 0.4 to 0.6 (200 to 300 PPM), then your water has enough of both calcium and magnesium, then make sure your watering (with or without nutes) is in the pH range between 6.0 and 7.0). My guess, the pH is too high (as tap water usually is). There are some pH down solutions that you can buy, you can also use apple vinegar to lower it too if you prefer something more organic. Good luck