So, if a typical and common chord progression for the Major scale is I IV V, but I wanted to use a mode to write a song so it doesn't sound so obvious - say, the Phrygian mode - I would use the same basic pattern?
For example, were I to write a song in C Major based on that common progression of I IV V, then the chords would go C, F, G. But if I wanted to write a chord progression in the Phrygian mode, but with the same structure as above, I would just play Em, Am and Bdim and it would sound entirely different. But it would still technically be in the key of C.
And if I so wanted, when the verse returns to Em, i could use that point to branch off into a chorus in a different mode, like making the Em stand for the IV chord, and having a chorus made up of, again I, IV, V, but this time in the Locrian mode, where the chords would be Bdim, Em, F? Then I could find my way back to Phrygian via Em.
Is this sound theory?