Chāndogya Upaniṣad 6.4.4

6.4.4

yad vidyuto rohitaṃ rūpaṃ tejasas tad rūpam \
yac chuklaṃ tad apām \
yat kṛṣṇaṃ tad annasya \
apāgād vidyuto vidyuttvam \
vācārambhaṇaṃ vikāro nāmadheyaṃ trīṇi rūpāṇīty eva satyam \\604.4\

4. Whatever red form the lightning has, is the form of heat; whatever white, the form of water; whatever dark, the form of food. The lightninghood has gone from the lightning: the modification is merely a verbal distinction, a name. The reality is just "the three forms."