Standardization was important when the different electronics companies of the world (including Philips, Sony, JVC, Matsushita) were exploring and inventing various purposes on what they could do with the compact disc technology. It was much more than just allowing audio to be pressed on it. Philips took the lead in this process and developed, together with the other big electronics companies, the Rainbow Book standards. The Rainbow Books are a collection of CD format specifications. It started with the Red Book about the Audio CD (CD-DA (Digital Audio), and it also describes CD-Text and CD+Graphics. The Red Book origins from 1980. After that, the Green Book was the second one in 1986 and describes the functions of the CD-i. In principle, the basic outlines of how CD-i would look like was indeed presented in 1986, five years before the official launch.
The Yellow Book, specified in 1988 was about the CD-ROM format. In 1990, it was followed up by the Orange Book, which was actually a two-chapter edition about CD-Recordable and CD-Rewritable. 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.
In 1992, the Photo CD standard was described in the Beige Book (see below, it is supposed to be beige)
Below: The Orange Book standards
The White Book in 1993 described the Video CD standard as well as CD-i bridge a bridge format between CD-ROM XA and the Green Book CD-i, which is the base format for Video CDs, Super Video CDs and Photo CDs.).
While some of these specifications are available online, these were originally protected quite strongly. You were able to order a copy here. For instance, the Green Book CD-i specifications is also available on ICDIA.
[Thanks, Seventy7 for the scans]