SB 4.14.45

तं तु तेऽवनतं दीनं किं करोमीति वादिनम् ।
निषीदेत्यब्रुवंस्तात स निषादस्ततोऽभवत् ॥४५॥

taṁ tu te 'vanataṁ dīnaṁ
kiṁ karomīti vādinam
niṣīdety abruvaṁs tāta
sa niṣādas tato 'bhavat

tam—unto him; tu—then; te—the sages; avanatam—bowed down; dīnam—meek; kim—what; karomi—shall I do; iti—thus; vādinam—inquiring; niṣīda—just sit down; iti—thus; abruvan—they replied; tāta—my dear Vidura; saḥ—he; niṣādaḥ—of the name Niṣāda; tataḥ—thereafter; abhavat—became.

He was very submissive and meek, and immediately after his birth he bowed down and inquired, “Sirs, what shall I do?” The great sages replied, “Please sit down [niṣīda].” Thus Niṣāda, the father of the Naiṣāda race, was born.

It is said in the śāstras that the head of the body represents the brāhmaṇas, the arms represent the kṣatriyas, the abdomen represents the vaiśyas, and the legs, beginning with the thighs, represent the śūdras. The śūdras are sometimes called black, or kṛṣṇa. The brāhmaṇas are called śukla, or white, and the kṣatriyas and the vaiśyas are a mixture of black and white. However, those who are extraordinarily white are said to have skin produced out of white leprosy. It may be concluded that white or a golden hue is the color of the higher caste, and black is the complexion of the śūdras.