Color::Green is an enum variant while { Color::Green } is a (constant) block (expression) that evaluates to an instance which is used as a generic type parameter.
The difference might be more easily understandable if we look at an enum variant that holds a value, where the syntactic differences between variant and instance constructor are more clearly visible.
Color::RGB(u64, u64, u64) vs { Color::RGB(10, 20, 30) }
