Not every bar in an 8- or 16-bar progression needs a different chord; it’s about balancing variety (things that are different) with unity (things that repeat).
A great tool for adding a different color in a chord progression is to include one or more half-diminished seventh chords (sometimes just called half-diminished chords).
The most common and most useful of these chords is the seventh, which you can use pretty much anywhere just to add a slightly different color to a chord, or in some types of progression to give the sequence of chords some extra thrust, because the added seventh is a mild dissonance, or clash, that makes the chord sound like it wants to move somewhere.
Chromatic alterations, such as sharps and flats, can be used to help create a more interesting chord progression in a piece of music. Ed Bell explains how to use these alterations effectively.