Maitri (Maitrāyaṇīya) Upaniṣad 6.29.p2

6.29.p2

etaduktvāntarhṛdayaḥ śākāyanyastasmai namaskṛtvā yathāvadupacārī kṛtakṛtyo maruduttarāyaṇaṃ gato na hyatrodvartmanā gatiḥ eṣo'tra brahmapathaḥ sauraṃ sdvāraṃ bhittvorddhvena vinirgatā ityatrodāharanti anantā raśmayastasya dīpavadyaḥ sthito hṛdi |
sitāsitāḥ kadrunīlāḥ kapilā mṛdulohitāḥ ||

Having spoken thus, Śākāyanya became absorbed in thought.

Marut, having done obeisance and shown proper honor to him, having attained his end, departed by the northern course of the sun, for there is no approach by a side-path here in the world. This is the path to Brahma here in the world. Piercing through the door of the sun, he departed aloft. On this point they quote :—

Unending are the rays of him
Who like a lamp dwells in the heart.
They're white and black and brown and blue;
They're tawny and of pale red hue.