Pale upper leaves - what is it?

Firstly, what is showing in the leaves now, is a direct reflection of what was happening in the root zone 10-14 days ago. Problems with new growth are generally attributed to a problem with micro nutrients, also known as immobile nutrients, such as iron, zinc, copper etc. NPK are macro nutrient or mobile nutrients. Problems with NPK start at the bottom first then progrss up the plant. Since NPK are mobile, the plant can use them wherever they are needed, such as at the new top growth. Flushing and small differences in lighting won't show any effects within a day or two. So, I would say, this plant is suffering from a deficiency of trace elements or micro nutrients. This is easily solved by givinga dose or three of a soluble trace element/trace mineral mix. These are available at any decent garden centre. Since you are growing in coco, I would presume you are using a cal/mag supplement? If so, upping the dose slightly may help, it could also be the first signs of a magnesium deficiency. Either way, giving some trace elements won't be bad, only beneficial.