Then there are the yugāvatāras, or the incarnations of the millennia. The yugas are known as Satya-yuga, Tretā-yuga, Dvāpara-yuga and Kali-yuga. The incarnations of each yuga are of different color. The colors are white, red, black and yellow. In the Dvāpara-yuga, Lord Kṛṣṇa in black color appeared, and in the Kali-yuga Lord Caitanya in yellow color appeared.
So all the incarnations of the Lord are mentioned in the revealed scriptures. There is no scope for an imposter to become an incarnation, for an incarnation must be mentioned in the śāstras. An incarnation does not declare Himself to be an incarnation of the Lord, but great sages agree by the symptoms mentioned in the revealed scriptures. The features of the incarnation and the particular type of mission which He has to execute are mentioned in the revealed scriptures.
Apart from the direct incarnations, there are innumerable empowered incarnations. They are also mentioned in the revealed scriptures. Such incarnations are directly as well as indirectly empowered. When they are directly empowered they are called incarnations, but when they are indirectly empowered they are called vibhūtis. Directly empowered incarnations are the Kumāras, Nārada, Pṛthu, Śeṣa, Ananta, etc. As far as vibhūtis are concerned, they are very explicitly described in the Bhagavad-gītā in the Vibhūti-yoga chapter. And for all these different types of incarnations, the fountainhead is the Garbhodakaśāyī Viṣṇu.