In this example of a praedicare in quid the subject 'purple' does not signify some ontologically independent being such as Socrates, Butane (as such representing butane molecules), an Ice crystal, etc.), but an ontologically dependent being, of which the corresponding term of first intention belongs in one of the accidental predicaments.
However, the way of predication in "purple[ness] is a color" is wholly that of predicating a genus of a species, and therefore it is a predicare in quid.