The purpose of the color coding is to give as much information as possible in limited space and to help you distinguish items based on their different features, such as how recently they were updated or their assigned priority levels.
In the context of Newzie, 20 different colors are used to present information about time intervals and importance levels, as shown below along with the sense of colors in general.
Given in picture above, 20 levels of color coding starts with color red, continues with orange, yellow and green respectively and ends with color blue. Color 'red' is the most active/hot color and it represents most recently updated or important items. The degree of recency and importance decreases as closing to the color 'blue', which is the least active/cold color in the given color schema.
Color coding is heavily used in the ListView of the Subscriptions Panel, as well as for each post to give you the sense that when they were posts.
To get more information about how to interpret time intervals and importance levels from colors, check out help topics under.