This could be quite a dangerous concept to state, yet when used properly, any note in the chromatic scale may be used over any chord change. Without delving too deeply into music theory, technically there are seven distinct pitches in a scale which are “legal” notes for a particular chord change. For example, if the chord is C major, all the white keys on the piano are in the scale, or more accurately, can be used to improvise on the C major chord. Now the five black keys, none of which are in the C major scale, can still be played as chromatic passing tones. This is a chromatic note (a half step away from a scale tone) that is played in between two scale/chord tones. For example, instead of playing scale degrees 1, 2,3,4,5 in eight notes, you may play 1, #1, 2, 3, and 5. Usually, the chromatic passing tone is on an upbeat, or the “and” of the beat, but this is not always the case. Playing a chromatic note directly on the beat and resolving rhythmically can be a great effect. The example used the chromatic passing tone in between scale degrees on and two, yet the tones in between two and three, four and five, five and six, and six and seven-in other words, all the black keys on the piano, may be played on a C major chord. To extend the idea, any chord, whether major, minor, diminished, half diminished, augmented, or the big one, dominant, has several chromatic notes that aren’t in the scale of the chord, yet can be played on those chords.
Just think how many chords there are. How many do you know? Yet there are only twelve tones in Western music, the twelve tones of the chromatic scale. Go to a piano, play a chord you like in the left hand, and with the right hand, start playing and running the scale that goes with that chord. Then, start adding the notes that are outside of the scale. Start simple at first, remember if it doesn’t sound “good”, or “right” to you, change when and where you play the outside note. Once you are comfortable with all of the twelve notes on a C major chord/scale, start the process again on another chord. I’ve found that practicing this concept systematically works well. Start with major chords; go to dominant or seventh (C7) chords, then minor, diminished, etc. Always create pleasing melodies, and be able to sing what you play. The great musicians can sing what they play, yet they can also play what they hear in their heads. Experimentation is the name of this game, so don’t be afraid!
As always, happy practicing!