The Blue Series contains 60 books - 30 sentence books in one box and 30 story books in another. This series introduces three letter words that your child can sound out. A few short "sight words" are introduced to allow for some simple stories.
The Yellow Series is next with 20 books including four letter phonetic words. Further sight words are taught.
The Green Series introduces double letter sounds like "oa" in "boat". This considerably increases the number of words that your child can sound out and solve for themselves.
The Red Series expands on the Green Series with more challenging combinations of sounds like "ay" in "play" or "a_e" in "gate". Understanding these rules is important in reading and later becomes essential in spelling.