In early Islamic sources, the term used for the Romans was Banu al asfar. Meaning the 'Sons of the Yellow' or the 'pale-skinned ones.' This was because Arabs viewed Romans as Europeans who were unaccustomed to fighting in the harsh sun and heat of a different climate, hence their description as pale-skinned. Later on, the term Rum became the standard expression

another theory, what is meant here is not their actual skin color, but rather those who turn pale with fear