There are several reasons why a fir tree like this would have yellow needles. 

Once reason may be seasonal needle drop.  

Deciduous trees, such as maple and ash, drop all of their leaves in fall. Though it largely goes unnoticed, evergreens also lose a portion of their foliage (needles) on a yearly basis. Seasonal needle drop on most evergreens occurs in fall.

Needle loss is most noticeable on white pines. As much as one-half of the needles on white pines may drop in early fall. Seasonal needle loss is less noticeable on spruces, firs, and other pines as they retain a higher percentage of their needles, but it is still common. Seasonal needle drop is uniformly distributed throughout the inner part of the evergreen. It is the oldest needles which are shed. The needles turn uniformly yellow or brown and drop to the ground.  What is picture in you rmessage does not seem to be uniformly distributed, so this may not be the issue, but is important to look into and rule out this potential cause.  No action is needed if the yellowing you are seeing is due to seasonal needle drop.

Another reason you see yellowing needles is due to environmental stresses.  This is the most likely reason for your issues.  It is common for newly planted trees to exhibit signs of stress due to lack of water or a sudden change in light levels causing leaf scorch or drop.  Check the soil moisture around the tree often.  If the original root ball or the surrounding soil are too dry, it could cause the yellowing of leaves/needles.  If conditions are hot and bright, especially in association with dry soil conditions, it is also possible that the leaves have received damage due to heat or a sudden change in light intensity.  Be sure to provide good consistent care to the tree and over the next several months to years.  The tree should be able to grow out of the issues and recover.  

It is possible that this may be a insect or disease problem.  Newly planted trees are under more stress making them more vulnerable to insect pests or disease issues that the tree would normally be able to ward off if it was more established and growing more vigorously. In most cases, any insect or disease issue is secondary to another problem like overwatering or improper planting. If you see insects or evidence of disease problems (such as a fungus growing on the needles) then it would be beneficial to diagnose and treat the problem.  If you suspect insect or disease issues (which is not likely based on what I can see in the photo you submitted), you can send a sample to the Iowa State University Plant and Insect Diagnostic Clinic to confirm what disease you are trying to manage. More information on how to collect and submit a sample can be found here: https://hortnews.extension.iastate.edu/pidc/plant

Environmental stress is the most likely culprit to the yellowing that you are seeing.  Provide good consistent care for the next year or two - making sure to pay special attention to watering when needed.

More information about decline in newly planted trees can also be found here: https://hortnews.extension.iastate.edu/faq/why-my-newly-planted-tree-dying

Utilize this information to evaluate your situation and pinpoint the potential cause(s) to the issue.

Happy gardening!

--Aaron