Kalki Purana describes the appearance of Bhagavān Kalki in this manner :
The complexion of the Lord of Laksmi was dark, like a tamala tree, and He was dressed in yellow garments He had lotus-like eyes, His arms reached to His knees, and His chest was very broad and displayed the mark of Srivatsa and Kaustubha gem.
Source : Kalki Purana ch 9
The very next chapter too says that Bhagavān Kalki is dark complexioned :
Padmavati's complexion was very fair and Lord Kalki's complexion was dark, like a rain cloud. In this way, they appeared to materialistic vision to be opposite one another. Princess Padmavati was dressed in blue garments, and Lord Kalki was dressed in yellow garments.
Source : Kalki Purana ch 10
Kalki avatar will take place in future, hence we don’t find much details about him, like we have for other incarnation of Bhagavān Viṣṇu.
Bhagavān Śrī Kṛṣṇa in Bhagwad Gita says :
yadā yadā hi dharmasya
glānir bhavati bhārata
abhyutthānam adharmasyatadātmānaṁ sṛjāmy aham
paritrāṇāya sādhūnāṁ
vināśāya ca duṣkṛtām
dharma-saṁsthāpanārthāya
sambhavāmi yuge yuge
Translation :
Whenever there is a decline in righteousness and an increase in unrighteousness, O Arjun, at that time I manifest myself on earth.To protect the righteous, to annihilate the wicked, and to reestablish the principles of dharma I appear on this earth, age after age.
Source : Bhagwad Gita ch 4 verses 7–8
