Mode mixture

If I am playing in the key of C maj, I can substitute with chords from the parallel minor; C minor...

But by playing in C major, I could also be playing in A minor because they contain the same notes...So with said, I could also substitute with chords from A major.

I know this is not common practice, but Cmaj/Amin contain the same notes, so you should be able to substitute with chords from both Cmin and Amaj, right?

Of course this would be a mess, but I mean when I am playing in Cmaj I might as well be playing in Amin and could substitute with chords from its parallel major too....

I understand that you choose to play in either Cmaj or Amin and substitute chords based on that decision. But the concept just crossed my mind

Any thoughts...?

What you are talking about is called modulation. I have a thread about it here: https://afpa.hooxs.com/harmony-f12/how-to-modulate-t198.htm

Enjoy

No - Im talking about borrowed chords...modal mixture