type color: enum { Red, White, Blue, };defines the values Red, White, and Blue. A variable of type color holds one of these values. Note that Red et al have global scope. You cannot define a variable or type with those names. (Also note that, as usual, the comma after Blue is optional.)
The only operations allowed on enumerations are comparisons for equality. Unlike C enuemrations, they do not have values or an ordering associated with them.
You can extend the set of values in an enumeration using redef enum identifier += { name-list }:
redef enum color += { Black, Yellow };