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In the dhyAna-mantRa provided before Vishnu-Sahasranaama, He is referred to as 'Megha-Varna' i.e. 'cloud-colored' (starting with 'shanantaakaaram').But in the starting part ('Suklaamvaradharam Vishnum'), He is mentioned as 'Sasi-Varna' (Moon-coloured). What is the reason of this difference? Also, the dhyana starting with 'sa-sankha-chakram' described Him as 'sa-pitavastram', i.e. His garment is yellow, but the beginning of the Sahasranaama describes Him as Sukla-amvara-dhara' meaning His garment is white.

What is the reason of these differences? (I know that He is also referred to as Suvarnavarna etc within the Sahasranaama, but these are not counted in His main dhyAna.) Could anybody throw some light on the differences?

Vishnu appears in different colors in different yugas. This is mentioned in Mahabharata Vana Parva (3.148). Here I am presenting the (edited) translation of van Buitenen -

Hanumān said: That Eon is called the Winning Throw, my friend, in which the sempiternal Law holds reign. In that age, that best of Eons, things are done, not left to be done. There the Laws do not lapse nor do creatures die; hence the name Krtayuga, which in time became equivalent to virtue. In the Krtayuga there are no Gods, Dānavas, Gandharvas, Yaksas, Raksasas, or Snakes, my friend, there is neither buying nor selling. The sounds of the Sāman, Yajus, and RC do not exist, nor is there human labor. Fruit appears when thought of, and the Law is relinquishment. At that junction of the Eons there are no diseases or diminishing of the faculties, no discontent, no tears, no pride, no libel, no strife, no lassitude, no hatred, no hostility, no fear, no suffering, no envy, and no jealousy. Then the supreme Brahman is the highest goal of the yogins, and the white Nārāyana (Vishnu) is the soul of the beings. Brahmins, barons, commoners, and serfs are well defined in the Krtayuga, and the creatures stick to their own tasks, The stages of life, conduct, knowledge, intelligence, and vigor are equally distributed, and the classes obtain their merit of Law by equally distributed activities. Being yoked to a single Veda, carrying out the prescribed rites with one and the same mantra, they are avowed to the same Law in their various Laws and single Veda. With their acts according to age conforming to the four stages of life and receiving the fruit without self-interest, they attain to the highest goal. The Law of the society of the four classes, conjoined with self discipline, is well defined in the Krtayuga, complete in four quarters, and sempiternal, This is called the Krtayuga, which transcends the conditions of the Three Constituents; now hear about the Trey (Treta yuga), in which the Sacrifice appears. The Law now is diminished by one quarter, and Acyuta (Vishnu)" becomes red. Men are bent upon truthfulness and devoted to the Law of rites. In the Tretà, sacrifices become current and all manner of Laws and rituals, now motivated by purposes and giving rise to fruit of acts and gifts. Given to austerities and donations, people do not stray from the Law in the Tretäyuga; they abide by their own Law and perform rituals. In the Eon of the Deuce (Dvapara yuga) the Law survives only half. Visnu becomes yellow, and the Veda fourfold. Some people know four Vedas, others three or two or one, while some have no hymns at all. While the Scriptures are thus broken up, the ritual becomes multitudinous; and, bent upon austerities and gifts, the creatures fall under the sway of the Constituent of Passion. Because the single Veda is no longer known, the Vedas multiply; and because there is now a collapse of truthfulness, few abide by truth. Many diseases strike those who have lapsed from truth, and lusts and disasters caused by fate arise, afflicted by which some men perform very severe austerities, while others, motivated by desires or the wish for heaven, hold sacrifices. Thus, having come to the Dvāparayuga, the creatures perish from lawlessness. In the Eon of Discord (Kali yuga), Kaunteya, only one quarter of the Law survives; and, having reached this age that is swayed by Darkness, Keśava (Vishnu) becomes black....