Yes, we usually think of plants being green. It sounds like you know that the green pigment chlorophyll is important in photosynthesis. What might be news is that there are actually many pigments that can trap light for photosynthesis.

If you live in a place where leaves change color in the fall, you have seen some of the colors of other pigments. Those leaves were green before. Then the tree stopped putting chlorophyll into the leaves, and the sunlight broke down the chlorophyll that was there. The other pigments were left behind. They were there all along, you just couldn’t see them.

The different pigments use different colors of light. You know the colors of the spectrum if you have ever seen a rainbow. We can tell which colors a plant is NOT using because those are the colors being reflected to our eyes. They are absorbing and using the other colors. So your average plant, like corn, is reflecting the green light and using the other colors. The purple cabbage is using the green and other colors, and reflecting the purple.

Light at the purple end of the spectrum has more energy than light at the red end. Why do you think purple is not a very common plant color? You might be interested in studying plant ecology or plant physiology.

The leaves of purple plants still have chlorophyll which looks green to us. So since they have chlorophyll, they can carry out photosynthesis. However, some plants have a lot of compounds called anthrocyanins which can be red or purple in color. If a plant has enough anthrocyanins, the purple will block out the green color even though the chlorophyll is still there. In fact, some apples are red on the side that gets sun and green on the side that’s in the shade. This is because in the shade they need to up their amount of chlorophyll to get enough sugar from photosynthesis so they appear greener. The reason that some plants are purple is that the color attracts bugs and animals that can help disperse the seeds of the plant. The purple color can also act as a “sunscreen” so that leaves don’t get to much sun.

Cabbage leaves are part of the flower, not the photosynthesizing part of the plant. Also, just because they have anthocyanin pigments that make them purple, they still also have chloroplasts with chlorophyll as well, just less of it. It's not the green color that allows plants to photosynthesize; it's the presence of chlorophyll, which just happens to be green, which is why most plants are green without any other pigments.