Why are the Bible book colors lighter or darker on JW library?

[The Librarian]

This question was just sent in to me.

[Evacuated]

Just a simple color coding based on a traditional grouping of Bible Books according to type, not chronology or authorship.  This is not the only grouping in existence, but it is a commonly used version. Others may care to comment in more detail on the history of the particular grouping as used in the New World Translation, but  In the case of the Hebrew Scriptures, the grouping dates back at least to the Septuagint version. 

The alternate colours make it easy (for some) to find a particular book in the list of 66,  (if you have an idea of the groupings of course).

Pentateuch or Law (5 books): Genesis to Deuteronomy

Historical (12 books): Joshua to Esther

Poetic or Wisdom (5 books): Job to Song of Solomon

Prophets (17 books): Isaiah to Malachi

Gospels (4 books): Matthew to John

Acts (1 book)

Letters (21 books): Romans to Jude

Revelation (1 book)