Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.2.4

13.4.2.4

atha hovāca sātyayajñiḥ trirūpa evaiṣo'śvaḥ syāttasya kṛṣṇaḥ pūrvārdhaḥ śuklo'parārdhaḥ kṛttikāñjiḥ purastāttadyatkṛṣṇaḥ pūrvārdho bhavati yadevedaṃ kṛṣṇamakṣṇastadasya tadatha yacuklo'parārdho yadevedaṃ śuklamakṣṇastadasya tadatha yatkṛttikāñjiḥ purastātsā kanīnakā sa eva rūpasamṛddho'to yatamo'syopakalpeta bahurūpo vā dvirūpo vā trirūpo vā kṛttikāñjistamālabheta javena tveva samṛddhaḥ syāt

But Sâtyayagñi said, 'That horse should be of three colours, its forepart black, its hindpart white, with a wain for its mark in front;--when its forepart is black it is the same as this black of the eye; and when its hindpart is white it is the same as this white of the eye; and when it has a wain for its mark in front, that is the pupil: such a one, indeed, is perfect in colour.' Whichever of these, then, should be ready at hand, either a many-coloured one, or one of two colours, or one of three colours with a wain for its mark, let him slaughter it: but in speed it should certainly be perfect.