Rishi Karabhajana told King Nimi in the Srimad Bhagavata Purana that in each yuga (eon as per Hinduism), Bhagavan Vishnu was worshipped in different forms and in four different colors – white, red, green and black.
Vishnu In Satya Yuga
In the Satya Yuga, Vishnu is white in color. He has four hands and covered his body with bark of trees.
In this yuga, living beings led a peaceful life by helping each other and not harming any living being. Living beings existed through cooperation. All living beings had control over their senses. They performed various austerities and merged in Bhagavan Vishnu.
In the Satya Yuga, Bhagavan Vishnu had ten names: Hamsa, Suparna, Vaikuntha, Dharma, Yogeswara, Amala, Iswara, Purusha, Avyaktah and Paramatma.
Vishnu In Treta Yuga
In Treta yuga, Vishnu is red in color, with hair of a golden hue. He wears golden color garments. He has four hands and there were three folds in His stomach. He has in His hands sruk and sruva – wooden ladles used for pouring ghee into the sacrificial fire.
People were adherents of dharma and were knowledgeable in the Vedas and praised Him through Vedic hymns. But there was selfishness and lack of cooperation among living beings.
Vishnu had eight names in Treta Yuga: Vishnu, Yajna, Trishnigarbah, Sarvadevah, Urukramah, Vrishaakapihi and Urugaayah.
Vishnu In Dwapara Yuga
In Dwapara yuga, Vishnu is green in colour and wore silk garments. He held in His hands weapons like the conch, discus and mace. In His chest was the Srivatsa. He was adorned with the Kaustubha gem. In this yuga, He was worshipped in four forms: Vasudeva, Sankarshana, Pradyumna and Aniruddha. He was known by the following names: Visvesvara, Visvaroopah and Sarvabhootatma.
In this yuga Adharma flourished as people had no control over their senses. There was hoarding of wealth. The strong tried to control the weak. Property and wealth were honored not knowledge.
Vishnu In Kali Yuga
Kali yuga, Vishnu is black in color. Blind with desire, people cannot see him. Adharma flourishes. There will be hoarding. Non-believers will flourish. There will be worship of false gods. People who follow dharma are seen as weak and fool.
In this yuga, Vishnu is praised through his names or Nama Sankirtana. Through Nama Sankirtana, people can get worldly benefits and also attain moksha. Because of this, the sages of Satya yuga will desire to be born in Kali yuga, so that they can sing His praises.
In Kali Yuga, Vishnu will be worshipped in the form of Krishna and Rama.