What's up Thundercat
*It's hard to tell color from the red output of the HPS, so yellowing is more pronounced...
I can suggest a few things...
Depending on how high the light is from them, when I start them under a 600W HPS from clone I keep the light a good 3 feet over them, after they start shooting up, I lower the light...
It can't be that the pot is too big, I put my clones directly into 5gal buckets.
*-Watering-*
It looks like they are in the 2.5 gal nursery pots, when you water, put at least a quart through them, it should be enough for sufficient runoff.
Then wait until the pots are light, and water again in the same method.
-You probably did overfertilize them, they don't need nutes until they are at the 4-5th node from seed.
You say your mix is pro-mix and potting soil... Most potting soil you can purchase already has nutes in the soil, wo this might also be a problem.
*-Flushing-*
If you think you have excess nutrient buildup, (which may be you case, along with too much light, and a pH problem) a thorough flush should be done.
For a 2.5 gal pot, run at least 2 gals of plain pHed water through them, some suggest more, but 2 gals should be enough.
Follow that with a light feeding to replenish the soil with the proper amount of nutes.
Also, the proper pH for soil is
6.2
Water temps when watering should be 60 degrees - 70 degrees.
65 is best.
Also, room temp is best at 75 degrees,
max 80.
If you're using co2, best temp raises to 83-85 degrees.
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As far as your feeding, you didn't mention using Big Bloom, that's a big part of the veg chart for FoxFarm, if you're just using the GrowBig, you aren't getting all of the beneficial guanos and micros.
Definately start using it.
And the plants shouldn't be turning purple, that's a sign of deficiencies, or a pH problem. If you have a pH meter, (which you definately should buy if you don't have one) test the plain water, adjust it to 6.2, and flush the soil. Collect the runoff, and test the pH, if there's a change from 6.2 then there is a pH problem in the soil.
Pro-mix is mainly peat moss, which has a low pH, Dolomite Lime is used to raise the pH of it when you mix up the soil, it's too late now, but now you know...
I think that covers all the questions you have... Back the light off a bit, and water properly and they should come out of it alright.