The show's standard opening sequence interplays images of the glyph symbols alongside words representing fringe science topics, such as "teleportation" and "dark matter". Within the third season, with episodes that took place primarily in the parallel universe, a new set of titles was used, following a similar format, though tinted red instead of blue and using alternate fringe science concepts like "hypnosis" and "neuroscience". The difference in color has led some fans to call the prime universe the Blue one in contrast to the parallel Red one. In the third-season episode "Entrada", the titles used a mix of both the blue- and red-tinted versions, given that the episode took place equally in each universe. In the two flashback episodes, "Peter" and "Subject 13," a variation on the sequence, using retro graphics akin to 1980s Technology and phrases like "personal computing" and "genetic engineering," was used. For the dystopian Future third-season episode "The Day We Died," a black-toned theme, with more dire phrases like "hope" and "water," was used. The fourth-season premiere, "Neither Here Nor There" introduced an amber-toned title sequence with additional new terms that is used for nearly all fourth-season episodes. The fourth-season episode "Letters of Transit," which returned to the Future dystopian universe, and the subsequent fifth-season episodes, feature a cold-toned title sequence with phrases such as "joy," "private thought," "free will," and "freedom," ideas which have been lost in this Future. There is one frame in the opening sequence in which the words "Observers are here" flash very quickly, and the opening sequence must be paused to see them.