Using dynamic type as function parameter

I'm having an issue using a dynamic type as a function parameter in a script
file.

I dynamically define and create the enum first, and then attempt to define
a function with the enum type as a parameter. For example:

New-Enum "MyType" Red Blue Green
#(from
http://blogs.msdn.com/powershell/arc...owershell.aspx )

function MyFunction ([string] $param1, [MyType] $param2) {}

If this code is in a script file, I get the error "Unable to find type
[MyType]: make sure that the assembly containing this type is loaded."

If I define the type at the command line, then execute the script, it's fine.

Additionally, I've found if I redefine the function like:

function MyFunction ([string] $param1, $param2 = [MyType]::Red) {}

I don't get errors when executing the script. However, I lose the strong
type of the variable.

Any suggestions for how I can keep the custom types on the parameter?