I have read the music theory sections and understand some (but certainly not all) of it. However, I wonder if there are any rules to the substitution of color chords for triad or seventh chords. I guess another way of putting it is when is the best time to use an "add9" chord? Or a "sus4" chord, or "6" or "11" etc. Also are there any general rules when to substitute a chord in common chord progressions and when not? For example, if you have a standard I VIm IV V I progression, does this progression lend itself to a color chord substitution and what would be the rules for doing so?
Thanks!
Any progression can be the starting point for substitutions. But there are tons of ways to do them, no hard and fast rules, and as with all other things in music - if you break the rules and it works, they weren't really rules (although they still might be useful guidelines!)
Let's consider the three main types of chords: major, minor, and dominant.
For a major chord, the typical color tones are 2/9 and 6. The 2 gives you an "add 9" chord; the 6 gives you a 6th chord. The 4 isn't very useful as a color tone - because it's just a half step from the 3. But you can use a sus4 to either decorate a chord (as in D -> Dsus4 -> D) or as a transition (Dsus4 is D-G-A, which shares two tones with G, G-B-D; the A moves up to B, which is a "suspension" in the classical sense). You can also use the 7, but that's generally considered a chord "extension" rather than a color tone - you're using the next third to form a major 7th chord.
For a minor chord, 9ths aren't often used... because it's just a half step away from the b3 in the chord. I think m6 chords have a very tasty sound, and you can also use a m/11 - since the 4th is a whole step from the b3, there's no conflict. And m7 chords are a useful extension.
For dominant chords almost anything goes. The 9th is an extension, giving you a 9th chord. The 4th gives you an 11th chord (the 9th isn't required in the voicing), or a 7sus chord if you're dropping the third - that's the basic chord in quartal harmony tunes. Adding a 6th gives you a 13th chord - like the 11th, the other tones aren't essential for the voicing.
I wrote a lesson for GN a couple years back on chord substitutions that outlines some other possibilities: Chord Substitution
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