Even on Earth colonies would be unlikely to be targeted. Nukes are limited, valuable, and going to be expended on targets in order of priority. For instance Peurto Rico would be extremely unlikely to be a target because it's just a completely worthless target in spite of having millions of Americans. Even from a morale crushing perspective, Americans do not consider Puerto Ricans Americans, in the same way that 'Martian Americans' will not be considered Americans for those living stateside. You'd achieve nothing.