Scratch

I'm currently in the middle of creating a game with a bird's-eye-view. Basically, you'll be looking down on the main character and his adventures throughout. What I'm having a problem with is that I have many backgrounds(rooms) to enter and I can't use the scripts with
                               "if touching color _"
                             "change to background _"

With all the different backgrounds(rooms) I have planned for this project, I'd eventually run out of colors, not to mention the colors I've already used to paint the backgrounds themselves. I need a script that will allow me to go the edge of the background screen, whether it be the top, left, right, or bottom edge and once I reach that edge to go the appropriate screen that I have broadcasted for it.

For example,

                           "if touching left edge of background 1"
                            "change to backround 2"

Do I need to make a variable for this?
Any help would be greatly appreciated.