Did each of Miller, Mann & Edmund (planets in Interstellar) orbit a star that itself orbited Gargantua? Or did the accretion disk of Gargantua create the light we saw on the surface of the planets?
According to Kip Thorne's The Science of Interstellar, the light is supposed to come from the accretion disk, whose temperature is supposed to be rather "anemic" compared to the ones that have been observed around real quasars (which are thought to be supermassive black holes like Gargantua). And an object's temperature is related to the peak light frequency it emits according to Wien's displacement law, which means that instead of emitting primarily X-rays like quasars, Gargantua has a spectrum similar to that of our Sun, emitting mostly visible light. Thorne writes in chapter 9:
Now, "anemic" doesn't mean anemic by human standards; just by the standards of typical quasars. Instead of being a hundred million degrees like a typical quasar's disk, Gargantua's disk is only a few thousand degrees, like the Sun's surface, so it emits lots of light but little to no X-rays or gamma rays. With gas so cool, the atoms' thermal motions are too slow to puff the disk up much. The disk is thin and nearly confined to Gargantua's equatorial plane, with only a little puffing.
Disks like this might be common around black holes that have not torn a star apart in the past millions of years or more—that have not been "fed" in a long time. The magnetic field, originally confined by the disk's plasma, may have largely leaked away. And the jet, previously powered by the magnetic field, my have died. Such is Gargantua's disk: jetless and thin and relatively safe for humans. Relatively.
Then in Chapter 19 about Mann's planet, he confirms that both it and Miller's planet are lit by light from the accretion disk:
Mann's planet can't be accompanied by a sun on its inward and outward journeys because, when near Gargantua, huge tidal forces would pry the planet and its sun apart, sending them onward in markedly different orbits. Therefore, like Miller's planet, it must be heated and lit by Gargantua's anemic accretion disk.