c-hanging-colons-alist is a variable defined in `cc-vars.el'.
Its value is

*Controls the insertion of newlines before and after certain colons.
This variable contains an association list with elements of the
following form: (SYNTACTIC-SYMBOL . ACTION).

SYNTACTIC-SYMBOL can be any of: case-label, label, access-label,
member-init-intro, or inher-intro.

See the variable `c-hanging-braces-alist' for the semantics of this
variable. Note however that making ACTION a function symbol is
currently not supported for this variable.

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.