Ruby: colon before vs after [duplicate]

This has absolutely nothing to do with variables.

:foo is a Symbol literal, just like 'foo' is a String literal and 42 is an Integer literal.

foo: is used in three places:

• as an alternative syntax for Symbol literals as the key of a Hash literal: { foo: 42 } # the same as { :foo => 42 }
• in a parameter list for declaring a keyword parameter: def foo(bar:) end
• in an argument list for passing a keyword argument: foo(bar: 42)