What are cool piano chords?
This is obviously objective but, in general you have the major chords which sound happy, the sadder sounding minor chords. Then you have the ambiguous 7th chords.
Now come the ‘cool’ chords. if you start adding the 9th, the 11th and the 13th to a chord then you start getting more jazz like sounds. The 13th chords even start to sound a little impresionistic and were used by Satie and Debussy.