To explain the working principle it is sufficient to cover the disk with one of black paper that leaves uncovered the five sectors of the same colour; then, by turning the disk quickly enough the entire surface of the circular crown appears uniformly coloured with that colour. For that to occur it is necessary for each coloured sector to reach the position in which the previous sector was, describing a fifth of a revolution in an interval of time shorter than that of the persistence of vision on the retina, which is approximately one tenth of a second; thus the speed of rotation must be greater than two revolutions per second.
In such a condition, in fact, when one of the five coloured sectors reaches the position in which the previous one was during rotation, the physiological effects caused by the sector in the intermediate positions occupied, with no interruption, are present on the retina and the entire sector appears uniformly coloured; since this occurs for all five sectors, the entire crown appears to be uniformly coloured.
When Newton's disk is turned at a speed above that limit, this occurs for all seven colours and if these reproduced the seven basic colours exactly, even in terms of relative intensity, one would have the same sensation of whiteness that one has with the seven small mirrors.
In practice, this situation is impossible to reach and the disk, rather than white, appears grey.