The proposition ' whiteness is a color ' then is in fact equivalent to the proposition ' subject X, which happens to be white, is a subject that happens to be colored '. So here it is only a matter of one subject being said of another (as subalternate), while the accidents are not (or only concomitantly so) predicated of each other.
In the propositions given earlier the accident was mentioned and the carrier (of the accident) connoted. At least so we can interpret the subject of the proposition ' the white (thing) is a colored thing ', namely as : ' the white [accident, mentioned] (thing) [carrier, connoted] is a colored thing '.
On the other hand, in the proposition ' subject X, which happens to be white, is a subject that happens to be colored ' the subject (carrier, substrate) is mentioned and the accident connoted. But also here the predication is not about an unum.
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