Iranian is a citizenship not ethnic group. there had never been any specific ethnic or linguistic group called as Iran or alike. Pahlavi regime choose this name only because it spells similar to Aryan! they were contemporary with the NAZIsm you know.
second, Croatians are mostly descendants from the Ural-Altaic speaking Scyth whom got assimilated into the Slavic because of the cultural and religious influence of the Byzantine and its Slavic Orthodox church.
but the Slavs were originally a brown skinned indo-european tribe whom immigrated to the southern Europe from the Hindustan through the middle east at the pre-historic era. this must be the answer you seek.
original indo-europeans were NOT white. white race appeared from the Ural-Altaic origins. we now know that the ancient Germanic and over all northern Europe, Euro-Asia, central Asian and Caucusians were white Ural-Altaic speaking tribes NOT indo-europeans.
one other group of those immigrating indo-europeans were ancient Persians:
but their language is extinct at the area. current Dari (so called Farsi) is a blend of Turkic, Hindi and Arabic languages and has NO similarities with the ancient Persian language. even the Sassanian era language was totally different than either of Dari or Achamenid Persian languages.
during the Pahlavi regime the Dari speaking group made a lot of propaganda to buy out some ancient identity and historical back ground through fabrications and propaganda.this is called as Pan Iranism fascism. they claim connection with the long lost ancient Persians and try to confiscate the historical, cultural and linguistic traits of real nations like Arabic or Turkic people.
Scyth is the Greece pronunciation, original name was ASHKUZ which means symbol of love in Turkic.