15.13Borrowed chords often weaken classical function but enrich color

This is important.

In strict major-key harmony:

G7 → C

is strongly functional.

But:

B♭ → C

is more modal.

It can still sound good, but its logic is not the same dominant-resolution logic.

So ask:

Is this chord creating directional tension, or is it creating color?

Borrowed chords often do the latter.