Google says yellow between veins is caused by iron...

Your plants are fine brother ! They aren't suffering from anything . The main thing you wanna focus on when they are young like that is not over watering them . Your starting PH in a soiless mix in my opinion should be 6.1 6.2 and you would gradually move it up to 6.3 or 6.4 this will ensure that your plant will not suffer from nutrient lock outs . I can see that you are folliar spraying your plants although this is good for them I wouldn't do it every day I would do it every 3 days or so , reason being is that when they are young like that a lot of what they absorb is through the leaves , so do it sparingly when young like that so you avoid over watering issues . As they get a little bit older you can add a rooting stimulate like piranha or voodoo juice (anything like those products is fine don't have to be those ones) a chelation agent ( humic / fulvic acids) and very very very light dose of which ever Veg solution you have chosen . If you do these things you will have no problems in the veg cycle and you will have wonderful healthy plants . I hope I've helped a little bit :)