He is giving the example just like a diamond or a valuable stone, you'll find a different color. Sometimes you'll find red, sometimes you'll find green, sometimes you'll find some other color, the same stone. Similarly, Kṛṣṇa when appears, that is explained in the Bhāgavatam, He appears in different colors: sometimes in blackish color, sometimes in reddish color. These are explained in the Śrīmad-Bhāgavatam. And in this age Kṛṣṇa appeared in the yellow color, Lord Caitanya. That is explained in the Śrīmad-Bhāgavatam.
- kṛṣṇa-varṇaṁ tviṣākṛṣṇaṁ
- sāṅgopāṅgāstra-pārṣadam
- yajñaiḥ saṅkīrtana-prāyair
- yajanti hi su-medhasaḥ
- (SB 11.5.32)
This is explained, authority. There are many other authoritative statements about Lord Caitanya's becoming Kṛṣṇa Himself. Idānīṁ kṛṣṇatām. Śuklo raktas tathā pīta idānīṁ kṛṣṇatāṁ gataḥ (SB 10.8.13). When Kṛṣṇa appeared, the nāma-karaṇa, when Gargamuni was ascertaining His name, he said that "This child, He has other colors," śuklo raktas tathā pīta. Pīta means yellow. That means some other Kali-yuga He appeared as Lord Caitanya. Idānīṁ kṛṣṇatāṁ gataḥ: "Therefore He should be named Kṛṣṇa." These things are there.
So Kṛṣṇa's appearance does not mean that He has become different. The nice example is set here by Baladeva Vidyābhūṣaṇa: just like a valuable stone. You take your hand . . . in your hand, you find sometimes yellow, sometimes red, sometimes white, like that. So, tava ceti mat-sakhatvat tavanti janmāni tavāpi abhut. "And because you are My eternal friend, so whenever I appear, you also appear."