Colourblind people see colours differently because they do not have all the necessary information from the L, M and S cones. A simile would be to try to obtain different paint colours by mixing only two primary colours: we would never be able to obtain the full range of existing colours.
While deep sleep is characterised by low brain activity, brain activity is high during REM sleep, similar to wakefulness. If we wake up in REM sleep, it is very common for us to remember that we were dreaming. But why do we dream? Ferran Martinez-Garcia explains.