Rainbow body or Ja-lus, also known as the clear light body, certain realized beings achieved the transformation of their ordinary bodies into a body of light. In this process the physical body Dissolves into its natural state, which is called ja-lus.
Which is that clear light body as the elements of body purified they transform from their gross manifestation, body, flesh, bone etc into their pure essence as the five colors. The Colors are blue, green, white, red and golden yellow, as the body dissolves into these five colors a rainbow is formed, and all that remains of physical body are just fingers, nail and hairs just left behind.
This process done by only the person who practice dharma from basic till dzogchen ati yoga.
Teachings of Rainbow Body is taught directly by Guru Rinpoche (Padmasambhava).