If I am in C, I often use the bIII, bVI and bVII chords from the Cminor scale. However, in an A minor aeloian progression I can just as well use these same chords except since A minor is the i chord that means the 3 chords above will also need to change their roman numerals. So how do I number them?
EDIT: When using roman numerals for C major we have I, ii, iii, IV, V, vi, vii. Right? When we have roman numerals from A minor aeolian we have i, ii, bIII, iv, v, bVI, bVII right? Now, when we borrow from C minor like C, Ab, Bb, C those non diatonic chords also need roman numerals and would use I, bVI and bVII. My confusion comes when a song or part of a song is in aeolian mode. What if the Bb chord appears in A aeolian? Is it now a bII chord? The song is still in C major but there may be a bridge in A aeolian so how do I number the chords in the bridge?