This article is from the RCA SelectaVision VideoDisc FAQ, by firstname.lastname@example.org (Tom Howe) with numerous contributions by others.
The color of the caddy often indicates some information about the disc
inside. White caddies are the most common, and usually indicate a
monophonic, non-interactive disc, but some of the later stereo and dual
soundtrack releases came in white caddies, and some of these releases
appeared in both white and blue caddies. Blue caddies indicate the disc
is recorded in stereo sound or indicate the disc is a dual soundtrack
(A/B) title. Black caddies are far less common than white and blue, and
were intended to indicate an interactive disc, although Disney released
their seven "limited gold edition" titles in black caddies. Black
caddies were also used on some discs not intended for public
dissemination, such as RCA test pressings and dealer demonstration
titles. Some gray caddies also exist, but these are so rare that most
collectors have never seen one. RCA intended to use gray caddies for
their industrial training disc program, but this program was only
implemented to a limited degree.