Though not the best position for prolonged meditative contemplation, Shavasana dulls the mind enough that the discernment necessary to achieve deeper meditative states becomes more difficult. (en.wikipedia.org)
The colors are also commonly seen by those who can achieve very deep meditative states. (activatemychakras.com)
When brain activity in this area is naturally reduced — for example, when we drop off to sleep or fall into a deep meditative state — our sense of where the body ends is appropriately minimized. (discovermagazine.com)