light green evergreen
The symptoms can be caused by a number of things. Looking at your photos, it is hard to tell what may be going on s. First, I think the tree that is yellowing might be a spruce and not Balsam fir. Spruce prefer well drained sites and we see this symptom on trees where the soil is too wet. Second, examine the tree truck to make sure there isn't feeding damaged/wounding from voles, etc. Also, you may want to look at the tree roots to see if they look healthy.