[DougWebb]

Basically, each narrow-band filter gives you a grey-scale image. You can assign whatever color you like to each of them, and then mix them to get a color image. If you have filters for the Red, Green, and Blue wavelengths, and then apply Red, Green, and Blue colors of the same wavelength to the images before mixing them, that'll get you as close to a true-color image as possible. (That's essentially what's going on in your camera, for capturing an image and then displaying it to you.)

Red, Green, and Blue are used because their wavelengths match the wavelengths that the cells in our retinas are sensitive to. That allows cameras to approximate human vision. But scientists use lots of other wavelengths too in order to see specific things more clearly, like Juno's methane filter. Every chemical gives off specific wavelengths of light when it releases energy, so filters that are tuned to those wavelengths make it easier to detect those chemicals. (I'm simplifying a bit here.) The false-color images you see from these missions are designed to combine multiple filters (that aren't RGB) and mix them using contrasting colors so that you can still see highlights from each of the filters.