The short answer to your question is that your plants could be any colour, but that is a massive over-simplification of the processes that lead to plant colours being what they are. I'll go through some of the basics though to spell it out in more detail.
Visible light is only 0.0035% of the EMR spectrum
For a start, the plant colours we see today are based on the fact that we only see a very small fraction of the possible spectrum of light. The beauty of this for us is that it gives a massive differentiation between night and day for us, and if we could see in the entire spectrum the difference would not be that discernible at all. To apply this to your plants on your planet, given that the source of light isn't that bright by comparison to our star (albeit it's closer) then it's more likely that your plants will have evolved to capture far more of the spectrum than terrestrial plants. Also, your animals have likely evolved to see more of the spectrum as well, meaning that they probably see the plants in colours we simply don't have names for.
Plants will be the inverse colour to useful light
Ultimately, photosynthesis is an endothermic reaction. That means, that the plant is actually absorbing the light of certain wavelengths and harnessing the energy through chemical conversion. Sugars and oxygen are effectively so energetic when put together because that energy has been harnessed by the plant that way in the first place. Of course, what that means is that the plant won't reflect the energy it absorbs; only the light that it doesn't. It's that reflected light that gives it its colour. You already know that, but the general rule you need to remember here is that the darker the overall colour of the plant across the entirety of the spectrum, the more light it's aborbing for photosynthesis. Therefore, it's unlikely that your plant will be a light colour, especially white, in any segment of the spectrum as it would mean it's reflecting back far too much energy for photosynthesis to occur.
Earth Based Life Bias
Perhaps most importantly, all this assumes an evolutionary path on your planet similar to that of Earth. It's entirely possible that life would evolve down a different path entirely other than photosynthesising plants and animal 'parasites', consuming the plants and eventually each other to harness that energy the plants create. Certainly, if the planet doesn't get a lot of strong light it's just as possible that eyes don't exist on your planet and that animals get around with a sensitive and precise sensory package that relies on an equivalence of smell. In such a case, the plants may well be black because that way they can harness the entire spectrum, but may emit other chemical pheromones or beacons to let the animals know where they are, especially if they have flower analogues to pollinate, or fruit analogues to eat for seed dispersal. The point being, we can't know exactly that light or colour even matters on this planet in terms of the local ecosystem.
So, your plants could be almost any colour, but they're far more likely to be darker than lighter, and I'd argue that they're also more likely (especially given the tidal lock) to use a much larger percentage of the EM spectrum than our own visible light range, meaning that if there are creatures there to see them, they are probably some dark colour for which we could never possibly have a name because we wouldn't be able to see it properly in the first place.
