The Rainbow Body
According to Dzogchen folklore, the attainment of the Rainbow Body is the sign of complete realisation of the Dzogchen view. As Tenzin Wangyal Rinpoche (2002: p. 141) states: “The realised Dzogchen practitioner, no longer deluded by apparent substantiality or dualism such as mind and matter, releases the energy of the elements that compose the physical body at the time of death."
http://en.wikipedia.org/wiki/Rainbow_body
Rainbow body (Tib. འཇའ་ལུས་, ja lü; Wyl. 'ja' lus) — fully accomplished Dzogchen practitioners can dissolve their body at the time of death.
Through the practice of trekchö, the practitioner can attain the so-called ‘rainbow body’, in which the body becomes smaller and smaller as it dissolves, emanating rainbow light, and finally only the hair and nails are left behind.
Through the practice of tögal, the practitioner can dissolve his or her body into the ‘Light Body’ (Tib. འོད་སà¾�ུ་, ö ku), where the body transforms into light and disappears completely into space. This was done by Garab Dorje, Manjushrimitra, Shri Singha, Jnanasutra and Vairotsana.
Another accomplishment of tögal practice is the ‘Rainbow Body of Great Transference’ (Tib. འཇའ་ལུས་འཕོ་བ་ཆེན་པོ་, ja lü phowa chenpo; Wyl. 'ja lus 'pho ba chen po), where the master dissolves his body into rainbow light and lives for centuries in order to benefit others. Such was the case with Padmasambhava, Vimalamitra, Nyang Tingdzin Zangpo and Chetsün Senge Wangchuk.
http://www.rigpawiki.org/index.php?title=Rainbow_body
The Rainbow body has 7 colours like the Rainbow. The 7 chakras in our body system has the 7 colours, each charka one colour.