Can anyone explain the discoloration of the lower ...

Looking good so far! The PH fluctuations will cause issues like this on the leaves. PH fluctuation could cause lockout of certain nutrients, which doesn't mean add more, it means the plant can't access those nutrients. I would be very careful about adding any more nutrients, especially this early in your grow. Also, the more nutrients you have in your reservoir or watering jug, the less of an impact the PH up and PH downs will have on your reservoir. Good luck and happy growing! 💪💪

I think it's running out of root space for one and add nutrients then adjust Ph. Light coloration could from Nitrogen deficiency. You are at fourth week in little cup so lots of roots could have sucked all nutes allready from the soil. And that cup is full of roots. I'd transplant her and she will be fine soon. Hope this helps some, good luck with your grow!