ok, thanks for everyones input.
Its not nute burn,..as i ommited to say, that when i increased the nute levels, i did it only on 2 of the 4 pots, but the 2 diferent strengths of nutes, showed the same problem.
Also, on one of the pics you can see a little nute burn on the tips...and it looks quite diferent from the yellowing, the nute burn is a more orangy/crispy browny color.

If i can refer everyone to this..Marijuana Plant Abuse

..near the bottom, potassium/zinc/(mobile elements) or Sulphur(imobile element) What is this mobile/imobile elements thing ?

Im pretty sure its a deficiency of one, or a combination, of these elements..but why there is a deficiency when im using a 3 part food grade nute, is beyond me.

I feel that adding anything extra to the mix just shouldnt be necessary..is it ?