Background light and background colors

[TMR Design]

I'm really starting to understand lighting and metering and as my little home studio grows I'm thinking about backdrops. I've made some that look very nice and just drape them over something or hang them as best I can but I don't own and don't want to start buying stands, hardware and backdrops at this time.
I read in a few threads that people were using a background light or flash with a gel on it to change a neutral background (or white wall) into any color they wanted. I'm using 2 AB B800's as key and fill lights and would like to bring the 580EX in as a background light. I have the Rosco Roscolux color gel swatches and will be making some sort of gizmo to attach the gels to the flash but also keeping them off the lens of the flash. I've already melted one that way. This will alow me to attach a gel to the head of my 580EX and create a background lit in color and perhaps get some good effects.

So before I start doing this I was wondering if anyone had suggestions or tips for doing this, or if anyone does not recommend doing it and why?
Looking forward to your feedback.

[coreypolis]

Does THIS Help?

[TMR Design]

Yes it does help and thank you very much. Not only does it answer my questions but gives me more specifics on how to do it properly.
Now I'm excited to wire my 580EX into the lighting and try this.

In the photos from that link it appears that the light on the far right close to the background is the one with the gel, so I assume that based on the effect desired and the intensity of the flash, the light can come from the side and does not have to be centered behind the subject. I was wondering about that.

It makes sense that the subject to background distance should be large enough so the reflection of the colored light does not splash back onto the subject. Is that just trial and error as to that distance or is there a generalization you can make about subject to background distance?

[coreypolis]

pretty much trial and error with the distance thing. You can block it off if it becomes an issue.

the BKG light is the only one with a gel on it. The hard part is keeping the main and fill lights from contaminating the BKG and from the BKG light from spilling over onto the model. Just have to practice with it.

It doesn't have to be centered, but it can be difficult to get a even tone all the way across if not (actually even centering it can be a pain). Gradations are nice though

[TMR Design]

Originally Posted by coreypolis

pretty much trial and error with the distance thing. You can block it off if it becomes an issue.

the BKG light is the only one with a gel on it. The hard part is keeping the main and fill lights from contaminating the BKG and from the BKG light from spilling over onto the model. Just have to practice with it.

It doesn't have to be centered, but it can be difficult to get a even tone all the way across if not (actually even centering it can be a pain). Gradations are nice though

Yes I can see the grad effect in your example and I also see that the angles of camera and main light to background have been adjust to prevent the spillover of main light on to the background.