Color changes in leaves are common and occur because of changes in physiology during seasonal cycles, changes in water availability, changes is nutritiona level, insult from parasites, (insects, nematodes) and some reasons I haven't listed here. A slight color change after undergoing a temperature shock would not be a surprise. The usual color change in junipers that are exposed to freezing conditions is for the needle leaves to get a sort of bronze color. That is normal. The pigment is actually redish purple but mixed with chlorophyll it looks bronze. Yellowing of leaves may mean that the tree is too wet. In cold weather it is better to keep the tree damp but not wet. They generally survive cold better if they are on the dry side. Jack

Juniper needles turning a slightly lighter green, or a more grayish green usually means the tree is getting desperate for water. Waiting for the color change to water is not a good idea, since letting it go just an hour or two too long can cause permanent damage to the tree, often its death! Don't let more than the top 1/4 inch of soil dry out between waterings, because junipers have no forgiveness for drought when in a bonsai pot. I found this out the hard way--several times! Repeated light damage will result in a wimpy tree with lots of brown needles and tips on it.

Welcome to the bonsai world, ryanbonsai!
Kevin : )