In each millennium the Supreme Lord is worshiped differently. In the Satya-yuga the process of self-realization was meditation, and this process is taught by the white incarnation of God. This incarnation gave a benediction to the sage Kardama by which he could have an incarnation of the Personality of Godhead as his son. In the Satya-yuga, everyone meditated on Kṛṣṇa, and each and every living entity was in full knowledge. The process for self-realization recommended in the Treta millennium was the performance of sacrifice, and this was taught by the red incarnation of God. In the Dvapara millennium, Kṛṣṇa was personally present, and He was worshiped by everyone by the mantra:
“Namas te vasudevaya Namah sankarsanaya pradyumnayaniruddhaya tubhyam bhagavate Namah”
“Let me offer my obeisances unto the Supreme Personality of Godhead, Vasudeva, Sankarsana, Pradyumna, and Aniruddha”. This was the process of self-realization for the Dvapara age. In the next millennium – this present age of Kali-yuga – the Lord incarnates to preach the chanting of the holy name of Kṛṣṇa. In this age, the Lord is yellow (Caitanya Mahaprabhu), and He teaches people the love of God by chanting the names of Kṛṣṇa. This teaching is carried out personally by Kṛṣṇa, and He exhibits the love of Godhead by chanting, singing and dancing with thousands of people following Him. This particular incarnation of the Supreme Personality of Godhead is foretold in Srimad-Bhagavatam (11.5.32):
“Kṛṣṇa-varnam tvisaKṛṣṇam sangopangastra-parsadam yajnaih sankirtana-prayair yajanti hi sumedhasah”
“In the age of Kali, the Lord incarnates as a devotee, yellowish in color, and is always chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare. Although He is Kṛṣṇa, His complexion is not blackish like Kṛṣṇa’s in Dvapara-yuga but is yellow/ golden. It is in Kali-yuga that the Lord engages in preaching love of Godhead through the sankirtana movement, and those living entities who are intelligent adopt this process of self-realization”.
Source: A.C. Bhaktivedanta Swami Prabhupada (2012 edition), “Teachings of Lord Caitanya, The Golden Avatara”, Page 100 & 101