Skandapurana 6.3.146.83

6.3.146.83

tameva nīlamityāhurnīlaḥ pañcavidhaḥ smṛtaḥ .
yastu vaiśyagṛhe jātaḥ sa vai nīlo viśiṣyate .. 146.83 ..

that indeed they call the dark [bull]; the dark one is recorded as fivefold.
However, one born in the house of a vaiśya is especially distinguished as a dark bull.