I would like to weigh in on this. When you water at 6.7pH, you are already at the high end of the usable pH range. As that solution hits the buffers in the soil, and the plant uses up some of the acidic nutes you mixed in, the pH has no choice but to start drifting upwards. Quite a range you have there... from 6.7-6.8pH... and then you are out of the range recommended for EDTA chelated nutes. Any of the nutes that are most mobile from 6.2-6.7 pH will be ignored or drastically reduced in availability to the plants, even though plenty of it is in the mix. Setting to 6.5 - 6.7 CAN lock out several nutrients, especially the heavy metals that are mostly mobile at the lower end of the range.
So that being said, the yellowing here does look like a macronutrient deficiency... not fall colors... to me anyway. When you are maintaining temperatures in the summertime range in your grow area... why would there necessarily be fall colors? If you have let the temp drop 20 degrees F, then I would understand the plants going through a change, because I have seen it happening. I have found however, that if I am properly supplying the nutrients needed, the right temperatures, and I am bug free... I don't get yellowing at the end.