The right side of the :: operator generally needs to be a Type:
julia> 1::1
ERROR: TypeError: in typeassert, expected Type, got a value of type Int64
julia> foo(x::1) = 42
ERROR: ArgumentError: invalid type for argument x in method definition for foo at REPL[6]:1
So it’s reasonable to use a “type scope” to highlight the RHS of ::, which is what the Julia extenstion in VS Code does:
EDIT:
Though I see that it doesn’t work quite as well as one might like. Not sure if this is fixable:
Here you can see that typeof is assigned the support.type.julia TextMate scope, which is not quite right.
You’re probably right that, strictly speaking, any expression can be used on the right-hand side of ::, so we shouldn’t really color those expressions differently, but practically speaking it is pretty nice to give the RHS (which is usually a type) a different color.