Enums in TypeScript 0.9 are string+number based. You should not need type assertion for simple conversions:
enum Color{
Red, Green
}
// To String
var green: string = Color[Color.Green];
// To Enum / number
var color : Color = Color[green];
Try it online
I have documention about this and other Enum patterns in my OSS book : https://basarat.gitbook.io/typescript/type-system/enums