Garudapurana 2.6.20

2.6.20

pītaḥ khuraviṣāṇeṣu sa nīlo vṛṣa ucyate /
śvetavarṇo bhavedvipro lohitaḥ kṣattra ucyate //

and yellow on the hooves and horns, that is called a dark-blue bull. / A white-colored one belongs to a brahmin, a red one is declared for a kṣatriya,