So, let's say there's a white dwarf star that has recently been born (by recently, let's say about 3 million years ago) Now around that time, a rogue planet, about the same size and mass as earth, came into the solar system and was pulled by the star's gravity, forcing it to orbit the star in it's habitable zone. Now, this rogue planet had frozen water all across its surface when it first came, but after it entered the habitable zone of the star its ice started to melt and eventually oceans were formed. Now, let's say after billions of years, life starts to develop and eventually it finds it's way towards the land. Now, if plants evolved on that planet, what colour would they be?
Interestingly enough, probably green.
Plants don't need a whole lot of energy to function. If we ignore factors like the refraction of light in your planet's atmosphere, the radiation hitting your planet's surface will be far shorter in wavelength than that hitting Earth's. However, as found here, photosynthesis is all about capturing those higher-frequency electrons that are in the sweet spot of high energy, not destructive. It's about finding photon wavelengths that are energetic enough to trigger photosynthesis and not cellular disintegration.
You could say 'hey, my plants are extra-radiation resistant', but realistically your plants are only going to want those same red and blue photons. They may reflect some black body (which isn't in the visible spectrum)- outside of that, plants on Earth have no use for the more powerful UV rays that penetrate our atmosphere. Why would plants on another planet?
I'm totally open, however, to the notion of deep blue or black plants that use very high-energy photons in smaller amounts. Plants here need a balance of lower energy red and higher energy blue, meaning they reflect the super-average green. If your white dwarf is mostly emitting in <500nm wavelengths, then it makes total sense for your plants to move up the spectrum and try to get those UV and yellow photons, thus meaning they reflect the (now) super-average blue spectrum.
Hope it helps!