Nepal was the place where the system of Adi Buddha was originated. The Swayambhu Purana gives an account of the first manifestation of Adi Buddha or primordial Buddha in the form of a flame of fire or clear light (skt. Prabhashvara). The meaning of clear light is self-cognisant and unfabricated original wakefulness primordially present in the mind stream of all sentient being of the three realms. Since it is primordially present in all sentient beings it is called Buddha nature or Tathagatagarbha or Adi Buddha.
The nature of clear light which is Dharmakaya is not generally represented in image or sculptures. For Dharmakaya is not the form body. Dharmakaya aspect is depicted in a stupa or in a Caitya form only. However in Tibetan Nyingmapa tradition, Dharmakaya aspect is depicted in painting as Adi-Buddha Samantabhadra/Samantabhadri form.
Samantabhadra is blue in colour symbolising emptiness essence of our mind and Samantabhadri is white in colour symbolising clear knowing aspect of the mind. The unity of emptiness and cognizant aspect is thus depicted in male and female form of Adi-Buddha. Adi-Buddha never meant first Buddha or creator of the universe.
Samantabhadri is totally nude and without even ornaments symbolizing dharmakaya or the nature of the mind which is free of thought constructs or fabrication. It means that Dharmakaya can be realised only when we are free from thought constructs and perceive the emptiness nature of our mind directly.
Source: The Iconography of Nepalese Buddhism by Min Bahadur Shakya