Could be a slight nitrogen deficiency.. I wouldn't start feeding crazy amount of nitrogen.. Unless your in a tropical climate... If your growing season is ending like mine is in the next 2-3 months.... I'd just do like light feeding of nitrogen to not loose anymore foliage... This is kind of a exception to the yellowing.. if this was indoors I would say no concern... outdoors the plant can get all around full circle sun.. The earth being flat and all LOL :)