In Benham's Top, a completely black stimulation is alternated with black lines and a completely white image. The rate of alternation is easily visible which is a comprehensible way of saying that it is below the flicker fusion frequency. In other words, unlike fluorescent lights, television, and movies, we can see it blinking from one image to the next. Usually, the pattern is presented in a rotating image like the one on the page at the Exploratorium. However, what is critical for the demonstration, is the alternation of the patterns and that can be accomplished several ways. In all cases, the subjects tend to experience pale colors, called Fechner Colors (Brown, 1965).
According to Brown (1965), the standard elements are black lines on a white background and a completely black field. From the construction of most Benham's Tops that I have seen, the black field makes up 50% of time of each repetition of the pattern. Looking at the disk at the Exploratorium is typical in that the black phase if half the disk. If the black lines immediately follow the black period, the lines are supposed to be tinged with red and if a homogenous white period is interposed between the black period and the black lines the color tinge changes from green to blue depending on the length of the white illumination (Brown, 1965). Rate of alternation and level of illumination affect the perceived colors. It is also my experience that there is a large degree of individual variability in the colors observed.
There is no definitive explanation for this phenomenon. The site at the Exploratorium proposes an explanation based on the finding that that in essences to short wavelength or blue cone responds more slowly to light than the other two cones (See the information in this online textbook on color for more background on this topic, Kaiser, 1996). Other textbooks are more circumspect in ascribing a reason for the phenomenon which strikes me as wise.
Brown, J.L. (1965). Flicker and Intermittent Stimulation. In C. H. Graham, ed., Vision and Visual Perception (pp. 251-320), New York: Wiley.
Kaiser, P. K. (1996). The Joy of Visual Perception. Online: http://www.yorku.ca/research/vision/eye/.