In رَأَيْتُ وَرْدَةً حَمْرَاءَ, both the noun and its adjective are accusative.
وَرْدَةً has fatḥa with tanwīn as a fully declinable noun; حَمْرَاءَ has fatḥa without tanwīn as a diptote.
Similarly, in مَرَرْتُ بِزَيْنَبَ, Zaynab is genitive but bears fatḥa in place of kasra.
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Causes of Diptote Status — Femininity and Its Conditions
In رَأَيْتُ وَرْدَةً حَمْرَاءَ, both the noun and its adjective are accusative.
وَرْدَةً has fatḥa with tanwīn as a fully declinable noun; حَمْرَاءَ has fatḥa without tanwīn as a diptote.
Similarly, in مَرَرْتُ بِزَيْنَبَ, Zaynab is genitive but bears fatḥa in place of kasra.