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).
Another common way to create more interesting chords by altering the notes of the basic triad is to form what are called suspended fourth 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.
Another great way to create more interesting chord progressions is to use a pedal note or pedal point. A pedal note is a static bass note that sticks around for a few measures while the chords change over the top.