Each of these represents a level in Lerner's new "Early Bird Stories" series:
Stories in the 'pink' level (two per book) each start off with a beginning letter to trace, high frequency and bigger key words, tips for reading, and a related activity. Text is a bit like the old "Dick and Jane" books in its repetitive framework, but the illustrations help make for an engaging story, as well as providing visual clues to support a beginning reader. (I loved Nell's completely unrepentant expression!)
Red books are the next step up in reading level, but share the same features. Sentences are a tad more complex, and stories do not have the repeating framework. Most words are still easy to sound out phonetically, with a few exceptions such as "prince".
With the yellow series we move to one longer story per book. More consonant blends and occasional longer words like "inventions". No letter tracing or high frequency words at the beginning, but a short quiz at the end with lower-level comprehension questions. Pictures are also more intricate, and at least in this example, encourage more reading with signage that helps make sense of the contents of poor Mr. Ricket's shed!
Finally, we have the blue level. Definitely more complex sentences and vocabulary (I'm not entirely sure what "skidding" is, though - strange choice). Use of quotation marks and other opportunities to practice oral reading skills. Again, a quiz at the end with basic comprehension questions.
Overall I am pleased with the sampling I have seen, and plan to order the rest of the books in this series for our Junior Reader collection! While they aren't as engaging as, say, Elephant and Piggies, they also aren't as mind-numbingly boring as some of the leveled readers the schools seem to get with their reading programs. Boring a child is never a good way to turn him into a great reader! I expect these to circulate well.