Well the more common conclusion to what I'm guessing your asking is a tapered band... that being you can incorporate torque and acceleration into one band set... thick at fork end will provide torque while thinner at pouch end ( being stressed harder ) will help total velotcity... its not the rubber, its how you use it!
I think he is talking about using truly differerently colored bands. I have thought about it, but is really simpler to use different widths, lengths , single, double or triple strands and taperings of the same color band.
Or are you thinking about mixing different thicknesses of bands to achieve an overall thickness for a desired width of band ? Asda exercise bands come in three thickness and if , for example, you wish for a half-inch band width, then mixing the colours would allow you to have various volumes of rubber for the same band width. Harry.
if you mix diffrent thickness flatbands, the thin ones will break first. thus rendering the 'left over lifespan' of the thicker rubber useless i.e. when one band breaks you can discard the whole set, or at most use the leftovers as tying strips.
so id keep the thin gauge rubber for highspeed/ light ammo bandsets, and use the thicker ones for heavy ammo/ more longeavity.
just my 2 cents, and i am NO EXPERT.
if you really wanna know the why's n the hows, i suggest you ask Texshooter or Flatband...