Given the two alternatives you mention, I believe you are better off with Color.Blue
. While the numeric values backing the enumeration are entirely arbitrary, the colours at least have some meaning in your application. If you are uncomfortable with setting the value directly because there is extra meaning associated to the default you can always define it separately somewhere else:
let standardUniformColor = Color.Blue
//(...)
type MyClass() = member val Col = standardUniformColor with get, set