You should be able to use "Layer modes" like in photoshop.
Like, screen, multiply, add, etc.
I use After Effects for compositing, and Premier for editing, but I believe that premier has this function as well.
Check the Premier docs for "Layer modes", and/or "Compositing".
Good luck!
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com