From the image you sent it looks like most of the needles are a healthy green. Some yellowing of needles is normal as the tree sheds old growth. If the center of the tree has the yellow needles do not be concerned. If the tips of the tree branches turn yellow then the cause is most often too much water or not enough water. Is the tip growth light green and healthy or is that the yellowing of the needles you are concerned about on the tips of the branches? Feel free to send a close up of the problem and perhaps I could be of better help. Keep growing, Marianne Binetti