Do you consider the South Sudan color change to be a change? I believe no particular shade of blue was prescribed before, but a dark blue was by far the most common choice.

Related markets:
Will any UN country change its national coat of arms or emblem before the year 2030?
Will any UN country change its national anthem before the year 2030?
Will any UN country change its capital city before the year 2030?
Will any UN country change its official name before the year 2030?

https://en.m.wikipedia.org/wiki/List_of_sovereign_states_by_date_of_current_flag_adoption