The Rainbow Books are a collection of CD format specifications.
Orange is a reference to the fact that red and yellow mix to orange. This correlates with the fact that CD-R and CD-RW are capable of audio ("Red") and data ("Yellow"); although other colors (other CD standards) that do not mix are capable of being burned onto the physical medium. Orange Book also introduced the standard for multisession writing.
Scarlet color of this book is a reference to the Red Book, which defines original CDDA.
((cite journal)): Cite journal requires