Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.36

7.3.1.36

tadāhuḥ | yadretaḥ sikatā ucyante kimāsāṃ reto rūpamiti śuklā iti brūyācuklaṃ hi reto 'tho pṛśnaya iti pṛśnīva hi retaḥ

As to this they say, 'If the seed is said to be seed what is its seed characteristic?'--Let him say, 'white;' for seed is white;--or 'speckled,' for seed is, as it were, speckled.