The secret to gels is creating your background color first, then balancing your subject. Brooks Institute of Photography teaches a technique called "Chromosones" which is real simple. I will write an article about it soon, bottom line, you only need either a white or black backround.
First thing to remember is that a "pure" white background reflects 90% of the light that hits it and a "pure" black background absorbs 90% of the light that hits it. With that said, you need no other colored seamless papers, only Black and White.
Take your lights, set them up where they evenly light the background, then take a roll of slide film and shoot one frame each in 1/3 stop increments starting from the lowest aperture to the highest--set your shutter speed at sync-speed. In other words, slap a "red" gel on your background lights, shoot at F/1.8, (and 1/3 stop increments between whole F/stops), 2.8, 4.0, 5.6, 8.0, 11.0, etc., then change gels, do the same over and over again with each color of gel.
Then have your slide film developed, un-cut, slap them on a black matte board with an opening the size of the width and length of the film, and tape it in over the sprocket holes. Use these matte boards to slap on a light table to show your client the different shades of color you can reproduce with your backgrounds.
Now you have all the "shades" of each color of gel, then mark them, so say if you have a "budweiser red" at F/8, then you set your camera at F/8, make sure your subject is at least a few feet or more from the background, then light your subject at F/8 with no spill on the background and you'll have Budweiser Red for the background.
If you want American Express blue, and your tests, your light, indicate it's at F/4.0, then light your subject at F/4.0. If you want to control depth of field, then switch from white to black or black to white background which should change your F/stops to just the opposites. Make sure and mark the ground where you set the light so you can set the lights the same way every time, and set the power pak to your background lights the same way--I recommend you have a separate power pak for your main light and keep the background lights the same every time.
Works like a charm, you can make any color you want with Black and White backgrounds, including turning black to white and white to black. If you come to one of my workshops I'll show you how...thanks, rg sends!