I believe it's safe to assume the flora would evolve to absorb every photon they can
But let's begin with a disclaimer. Unless there's a source of heat and energy available on the rogue planet, it's highly unlikely anything other than bacteria would evolve — and probably not even bacteria. Heat and energy are the basis of life. If the planet spun fast enough to keep the core hot and volcanism active, then it's believable that the flora would draw most of it energy from those sources.
And when you think about that, the idea of leaves begins to fall out of favor. What the plant would want is strong roots. If stems, trunks, branches, and leaves did form, they would do so to pull energy and nutrients primarily from the atmosphere near the geothermal vents or, if the conditions were right, moisture from the air.
Knowing this, the idea of terrestrial flora simply doesn't conform. I can imagine fungi or massively willowy plants. Trees don't make sense at all (from the perspective of my short-of-research answer).
But, that doesn't answer your question
let's ignore all that and simply answer the question. What color would the leaves of most plants be?
If electromagnetic radiation is important to the plants in a similar way that it is here on Earth, then the most likely color would be black. No light would be reflected in any or most spectra. On a rogue planet where every photon would convey precious life-giving energy, I can easily believe not a single one would be reflected. They'd all be absorbed. Which means a human looking upon the plants would only see the flattest, non-reflective black.