The multidimensionality of the same term meanings in different texts

One of my students asked a question about the Yogayājñavalkya text. There is a shloka in the section on dhyana:

अथवा परमात्मानं परमानन्दविरगहम्
गुरुपदेशाद्विज्ञ्याय पुरुषं कृष्णपिंगलम्।

athavā paramātmānaṃ paramānandaviragaham
gurupadeśādvijñyāya puruṣaṃ puruṣaṃ kṛṣṇapiṃgalam।

Or when receiving a teaching from the Guru, the embodiment of the supreme soul, the highest bliss, having dark and red color.

What does it mean by dark and red color? Why these particular colors?

The meaning of combination of these two colors in relation to Divinity can be found in a number of sources, first of all in purāṇas, such as Padmapurāṇa, Skandapurāṇa, Liṅgapurāṇa.

In Liṅgapurāṇa, in the description of greatness of the five faces of Shiva, namely Aghora, Aghora Shiva has the characteristics of Dark and Red colors.

स तं दृष्ट्वा महात्मानमघोरं घोरविक्रमम्।।
ववंदे देवदेवेशमद्भुतं कृष्णपिंगलम्।। १४.६ ।।

In Nārāyaṇasūktam, or for example in Mahānārāyaṇa Upaniṣad (Ch. 23), the famous mantra is given:

ऋतं सत्य परं ब्रह्म पुरुषं कृष्णपिंगलम् ॥ उर्ध्वरेतं विरूपाक्षं विश्वरूपाय वै नमः ॥

To the Great Order, Truth, the Great Absolute, the person in the shape of the Universe, with extraordinary eyes, having dark and red color, the one whose sexual power is directed upwards, to him I offer my respects.

Numerous comments to that ancient Vedic mantra contain different interpretations, as for example the dark (on the right) is a symbol of Shiva and the red color on the left side is a symbol of the Goddess Uma (Umāmahēśvara).

तादृशं ब्रह्म स्वभक्तानुग्रहाय उमामहेश्वरात्मकं पुरुषरूपं भवति | तत्र दक्षिणे महेश्वरभागे कृष्णवर्णः | उमाभागे वामे पिङ्गलवर्णः |

There is also the interpretation saying that these are the colors of Shiva and Nārāyaṇa (Harihara). But whatever the interpretations are, the idea of integration of basic metaphysical categories can be traced in all of them.

Yājñavalkya as a Nātha yogin

There is an interesting explanation of the oldest source of the yoga doctrine found in बृहद्योगीयाज्ञवल्क्यस्मृति Bṛhadyogīyājñavalkyasmṛti (12.5):

सांख्यस्य कर्ता कपिलः परमार्थः स स
हिरण्यगर्भो योगस्य वक्ता नान्यः पुरातनः ||12.5 ||

sāṃkhyasya kartā kapilaḥ paramārthaḥ sa sa
hiraṇyagarbho yogasya vaktā nānyaḥ purātanaḥ (12.5)

Kapila, the founder of Sāṃkhya, is known as paramārtha. (Similar to that) Hiraṇyagarbha expounded the yoga doctrine, there is none more ancient than him.

However, in the Nātha Sampradāya, Yājñavalkya is known not only as a Vedic ṛishi, but also as a disciple of Brahma (Satyanāth), who in the form of Hiranyagarbha transmitted the knowledge of yoga to Yājñavalkya. And Yājñavalkya Nātha is mentioned on the lists of 84 Nāthas.

āgamarahasyam Ch. 17

In the chapter devoted to the yoga practice, namely, in the section of dhyāna, the elements of sexuality are described in a positive context. Unlike many formal celibate guardians, who in fact do not follow it themselves, there is not so much hypocrisy directly in the ancient texts. On the contrary, sexual power is interpreted as extremely important in spiritual evolution. With the correct perception it is capable of bestowing a higher yogic realisation.

अग्रतः पृष्ठतो मध्ये पार्श्वतोऽथ समन्ततः ।
विद्युच्चकितवद्भाति सूर्यकोटिसमप्रभः ॥ २९७ ॥

रतान्ते स्त्री यथात्मानं क्षणं क्वाहं न बुध्यते ।
रमणोऽपि न जानाति कोऽहं योगे तथा पुमान् ॥ २९८ ॥

agrataḥ pṛṣṭhato madhye pārśvato’tha samantataḥ ।
vidyuccakitavadbhāti sūryakoṭisamaprabhaḥ ॥ 297 ॥

ratānte strī yathātmānaṃ kṣaṇaṃ kvāhaṃ na budhyate ।
ramaṇo’pi na jānāti ko’haṃ yoge tathā pumān ॥ 298 ॥

(Yogin) is shining with thousand millions of sun rays evenly in front, behind, in the middle, in the center and at the sides. During orgasm, a woman (रतान्ते स्त्री ratānte strī) tries to realise: “Where am I?” (क्वाहं kvāhaṃ?), but can not realise it. A man at the moment of pleasure asks the question: “Who am I?” (कोहं? ko’haṃ?) and cannot find the answer, also a person in the yogic state doesn’t know who he is (i.e. his false personality).