Juniper Growth
You've been taking care of your juniper per the care instructions but still notice that you've got yellowing needles. The most common reason for a healthy tree getting yellow needles is due to lack of sunlight.

This does not mean that the entire tree is not getting enough sunlight but rather the foliage on the inside of the tree is not getting enough sunlight. Juniper bonsai trees, especially healthy junipers will have dense thick foliage on the outside tips of the branches. These tips if allowed to grow freely will begin to get full. As it gets fuller, the sunlight reaching the inside needles will start to decrease. As the inside needles get less sunlight, they will begin to turn yellow and die off.

Essentially, the tree is mimicking how it would grow in the wild by allowing the outer growth to strengthen while eliminating the inside older growth. If you want to maintain green color in the inside needles, simply keep up on your regular maintenance of your juniper tree by cleaning out excess growth in the outer foliage. Doing so will allow light to penetrate through to the inside needles and strengthen them.