Sri Aurobindo translates Verses 4 and 5, Chapter 1, Section 2 of the Mundaka Upanishad: “Kali, the black, Karali, the terrible, Manojava, thought-swift, Sulohita, blood-red, Sudhumravarna, smoke-hued, Sphulingini, scattering sparks, Vishwaruchi, the all-beautiful, these are the seven swaying tongues of the fire. He who in these when they are blazing bright performs the rites, in their due season, him his fires of sacrifice take and they lead him, these rays of the sun, there where the Overlord of the Gods is the Inhabitant on high.”
There are certain experiences which arise when a seeker takes up spiritual practices in earnest. The appearance of various forms of light to the inner vision is quite common and has been described in yogic literature as well as in Buddhist traditions. A study published in 2014 in Frontiers of Psychology, titled A phenomenology of meditation-induced light experiences: traditional Buddhist and neurobiological perspectives undertook to describe and codify the type of experiences that arise during the practice of meditation. The yogic tradition also speaks of 7 primary chakras or energy centers and as these chakras “open” and their energy begins to pour out, different colors and lights appear. The Mundaka Upanishad refers to 7 “tongues” of the fire, holding once again to the outer, external image while referring to an inner experience. The Upanishad itself refers to them as “rays of the sun” which speaks to the illlumination that occurs. There are seven “worlds” that correspond to the seven chakras, seven colors, seven forms of light. The aspiration of the seeker, in conjunction with the practice of meditation, helps bring about illumination in all the seven realms, the material world, the vital world, the mental world, the supramental, and the worlds of existence-consciousness-bliss. The spiritual experiences open new vistas for the seeker and help to take his awareness to the place where all the manifesting powers of the creation originate and find their source and powers of expression.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210