What comes next?

[chromos]

It reminded me of my college days, when we were taught that "guessing" the next member of an infinite series is not a problem from mathematics, but from psychology (just put yourself in the mood of the author of the problem and guess which of the popular series he probably had in mind ). Even if we knew the first million members, e.g. 1, 2, 3, ..., 999 998, 999 999, 1 000 000, the next one could be absolutely arbitrary, not necessarily 1 000 001.